The medium Cupid was first released in February of 1998. The name "Cupid" comes from being released in February near Valentine's day. There were about a dozen early prototypes with ungrooved handles and without serial numbers. Here is a picture of one of those first dozen prototypes. This one has a chisel ground blade, but I don't know that they all had chisel ground blades.
The production models of the first release medium Cupids had grooved handles and serial numbers. All of the pieces that I've seen had gray anodized handles and double ground blades. I don't recall seeing any that were serrated. From what I've seen, serial numbers were in the 80xxx to 81xxx range. All of these first release Cupids had hand ground blades, as the Daltons did not get CNC grinding equipment until about April of 1999. Here's a picture of the production version.
The dimensions are:
Overall length: 8 - 1/2"
Handle Length: 5 - 1/16"
Blade Length: about 3 - 7/16"
Handle Thickness: 0.485"
Blade Thickness: 0.125"
A few of these had the Proto marking. Here is a picture of a Proto, courtesy of Brent (Polomax). It has the grooved handles, serial number and a double ground blade.
There were a few made with full satin blades:
By early 1999, the original run of Cupids had dried up and people were screaming for more. Original Cupids were bringing premium prices on the secondary market. In response to the outcry for more Cupids, Mr. Dalton made another batch of Cupids and released them in July of 1999. The second release versions all had black anodized handles and chisel ground blades. They were available with both plain and serrated blades. It appears that the serial numbers may have started at 00001. Both of my second releases are in the 002xx range. Here's a picture of a second release Cupid.
The third release Cupids were different in that they had a safety and double ground blades. These were released around November 1999. They were available with both gray and black anodized handles. I don't recall seeing any with serrated blades. The serial numbers appear to have started at 20000. Both of mine are in the 201xx range. I've seen at least one of these with a PROTO marked blade. Here's a picture of a third release Cupid.
The safety was changed to a button type safety on the fourth release Cupids, which became available in about March of 2000. I think that all of these had black anodized handles and double ground blades. They were available with both plain and serrated blades. Mine have serial numbers in the 203xx to 204xx range. I'm assuming at this point that the serial numbers may be a continuation of the third release Cupid numbers. Here's a picture of a fourth release Cupid.
If I was going to carry an OTF in my pocket, this is the one I'd carry. As far as I'm concerned, a safety is an absolute requirement for carrying an OTF in your pocket. The button safety on the fourth release is easier on the thumb than the safety on the third release.
As always, thanks to the Daltons for providing reference information and reviewing this post. Also, thanks to Brent for reviewing and providing the picture of the first release Proto.