Can anyone tell me any detailed information about this American Arms Derby 2 barrel 20/28 GA. Set?
Alvin,
The gun you pictured is a Zabala Hermanos (ZH) model Derby, two barrel set, 20 and 28 gauge. Here is another (one of mine as chance has it):


There were several variation of these imported. Mine is a DT gun; the one you picture has a single trigger. Could be a SST, or a SNST – both variations exist.
Mine was built as a medium game gun, but on the heavy end of the medium game gun range. Barrel walls were thick enough that I could have it cut for regular choke tubes, and didn’t have the expense of thin wall tubes. It has hand detachable locks, lock “engraving” is stamped and the lock is a back action lock work:


These are good, solid, working man’s sidelock shotguns.
The other “Derby” model shotgun, also imported by American Arms, is a different gun entirely. That gun was made by DIARM, top of barrels marked ‘Sarriugarte’, water table and barrel flats have the ‘BEST’ maker’s mark of Erbi, and they came in a box marked AYA, and DIARM. All those different markings have confused some folks into thinking these guns were actually made by AyA, or Erbi, or Francisco Sarriugarte. Nope. All three of those makers were out of business at the time, having joined DIARM.
The DIARM Derby is, IMO, a higher quality gun as it has more skilled hand labor in it (and hand cut engraving). But there’s nothing wrong with the ZH Derby, and the ZH may actually prove to be more durable in the long run.
I rather suspect American Arms began importing the ZH Derby after DIARM closed and it became impossible to get any more DIARM Derby shotguns.
Hope this helps.