Hi Stan:
The Model 4 had a round double-lock button. The Model 400 had a square one.
This whole model thing gets confusing with Peerless products, and what collectors call things as opposed to what the manufacturer does. You probably are aware of most of what follows, but others may not.
Until the NIJ standards were adopted, Peerless never used model numbers; they just called them "handcuffs."
As best I know, starting with Dick Norman, the original Peerless cuff using a bit key that double-locked with the key was called, among collectors, Model 1. The next lock mechanism, using a bit key but with a double-lock button was Model 2. The next lock mechanism, using a barrel key and button double-lock was called Model 3. When the 2,759,349 lock mechanism was introduced in the 1950s, it was, at the time among collectors, called the "New" model, which with the passage of time, began to be called Model 4.
And that's where it gets confusing. Production-wise, it went from Model 1 to Model 2 to Model 3 to Model 4 then back to Model 3. However, all of these had the round double-lock button.
With the Model 3 being back in production on the second go around, the NIJ standards were then introduced which required that handcuffs be stamped with a model number. So, Peerless denominated the standard chain handcuff the Model 300, and the hinged version (using the Model 3 lock mechanism) as Model 301 and began stamping them as such.
When Peerless came out with the flash in the pan scintered steel handcuff and lock mechanism with a square double-lock button, that got denominated as the Model 400. When they went back to the old way of manufacturing using rivets to hold the cuff body together, it was denominated the Model 500.
There was a Model 600, but it never went into production beyond the prototype stage.
http://www.network54.com/Forum/261154/m ... +handcuffs
When the raised rivet design came along, Peerless called it the Model 700.
