Thank you thumper for your usual insightful analysis! I've collected 55's for some time, and yes, there's more going on than just an HW 50 with a barrel lock. Many subtle and interesting tweaks to its details.
Here is the unique piston seal detail noted above. A similarly sized Diana seal is on the left, the other four are from HW 55's. Instead of the solid center piece typically used in leather cup seals, there's just a flat washer (three are metal, but on the right is a plastic one which disintegrated). If the chamber is kept well-oiled, the leather eventually wraps around the front of the washer to form a thick pad.
I assumed the first one I saw was a bad amateur re-seal, but after finding half a dozen it's obviously part of the OEM design to soften the firing cycle! Later 55's went to a more normal seal...as do I when I find the older ones.
The 55 is not a powerhouse, but you can replace the original target spring with one from the old HW 50, or an aftermarket equivalent, to get it up close to 700 FPS with light pellets. I actually rather prefer the quicker, crisper shot cycle this gives.
This old ad touches on some of the HW 55's interesting details, but you have to roll your eyes at some of the advertising claims! Would love to see those "laboratory controlled tests," eh...
