Evolution of the VM-68

When the VM-68 was first introduced it was known as the PMI-3. The PMI-3 was manufactured by Sheridan and then distributed by Pursuit Marketing Incorporated (PMI). About two years after it was introduced the name was changed to VM-68. Why was the name changed from PMI-3 to VM-68? Well, from what I've gathered so far the name change is a direct result of Sheridan being sued into bankruptcy and then getting bought out by Crosman. Crosman then changed the name because they no longer needed the 'PMI' in PMI-3. After the Crosman went with National as their distributor.

One very common misconception is that there is a difference between a PMI-3 and VM-68. This is not true. Many changes were made during the PMI-3's/VM-68's lifespan, however, none of the changes were made as a direct result of the name change. The name change was a marketing change only.

One of the most obvious differences between the very early PMI-3s and later PMI-3s/VM-68s is that the early models did not have ambidextrous cocking slots.
Along those same lines, the cocking knobs were different. Early PMI-3's had a separate cocking knob and safety knob.
To accommodate the use of two separate knobs, the cocking slots themselves were much longer. In the picture we see a VM-68 on the top and a PMI-3 on the bottom. The VM-68's cocking slot is 2-3/4" long as opposed to the early PMI-3's 4-5/16". Early PMI-3's had a VERY long firing motion compared to a VM-68.
Internally early PMI-3's are a little different from later PMI-3's/VM-68's as well. Here we see the size difference between an early hammer and a Cooper-T hammer (which is the same size as stock VM-68 hammers, I just didn't have a stock hammer handy when I took this picture).
To make up for the shorter hammer, the bolt had to be longer. The bolt was also white anodized and not black like the later models.
There was a brief transition period between the long early bolts and the now standard short bolts where the bolts were still anodized white. The bolts were later anodized black along with everything else. They probably switched to black to streamline the anodizing process. Instead of anodizing the bolts separately they could just do everything at once.
Early trigger assemblies were a little different than current packs. Early set ups had an additional trigger pin. I haven't looked into this much at this point so I'm not sure what the mechanical difference there is, if any, between the old and new trigger assemblies.

Early PMI-3's had aluminum side rails instead of the later plastic ones. These side rails, as well as all other aluminum parts, had the camo pattern anodizing to match the gun body. Shortly after Sheridan started production they went with solid black anodized side rails.The gun body was the only part anodized with the camo pattern and all the other parts were simply anodized black.

So far I've noticed four different types of side rails. There were the original camo ano aluminum rails, the all black ano rails, the first plastic rails with Racine, WI on them, and the later plastic ones with Bloomfield, NY on them.

Earlier PMI-3's and VM-68's have a different pattern in the ASA. Early versions have a 4 hole pattern whereas later versions have a 6 hole pattern.
Another small change was the addition of a retaining screw in the feed block. The screw was added to help hold either the older style elbows or the brass insert in place. Thanks to bored383 for pointing this out.
The most obvious way to see how old your VM is, is to look at the serial number. To the best of my knowledge, from 1990 to the end of 1992 Sheridan used sequential serial numbers with all there guns. PMI-3's and VM-68's have a six digit serial number stamped on the right side of the gun body under the feed adapter. Looking at the picture, 000163 is a very low number and this gun was made very early in the production run. The highest sequential serial number I've seen so far is P142846. If you have one higher, please let me know.

Starting in 1993 Sheridan started using a 9 digit serial number. The first 3 digits indicate the month of manufacture and the year. Example: 693 would be June 1993. However, I have seen variations of this. Two VM's I have start with a 'D'. I would assume this means the gun was made in December, but I haven't verified that yet.

I'm not sure when Sheridan stopped doing this but older PMI-3's and VM-68's had backblocks that where tapped half way through (10-32 thead). The idea was that you could turn them around and then bolt on an after market stock.
At some point Sheridan added a notch in the feed block. This helps the shooter see if there is paint going into the breech.
Starting in about 1996 Sheridan made a few small changes to the VM-68. One part to be changed was the trigger. Older triggers are chrome and the later triggers are black. The main difference is that the black triggers have a small lip on them and they don't require the use of the sear stop (item number 7 in this diagram). The only bad thing is that if you want to put a 'Cocker style trigger shoe on you need to grind off the small lip first.
Another small change Sheridan made to the VM late in it's production run was the shape of the grip lug. Pre-1996 VMs have a rounded lower face whereas 1996+ models have a flat lower face. Why the change? I don't know for sure but it might have been to reduce production time while milling the piece. Straight cuts are easier than curved cuts.
Yet another change made towards the end of the VM's production run was that the ASA was drilled deeper. This may have been done to lighten the body a bit. This also created an internal expansion chamber although it's not too effective due to the limited surface area.
Another small change with late run VMs is the transfer port between the valve and bolt. Older VMs had a ~11/64" transfer hole and was pluged on the top of the body with a 10-32 x 3/16" set screw. This was changed to ~13/64" and pluged with a 1/4-20 set screw. Although this does provide better flow through the gun body, that is probably not why it was done. This may have been done to streamline production. The rear screw, the big one that goes through both back blocks, is also 1/4-20. Rather than switch tooling between the drilling and tapping of both holes they could do them both with one set of tooling instead of two. Picture courtesy bored383.

Possibilities: These are things that Nick Brassard pointed out to me that we haven't been able to 100% verify. If you can verify any of these, please let me know.