Darn.
Just a fun fact. I have a Winchester 21 3 barrel set with 410, 28, and 20 gauge barrels and all have the same fire pin spacing. Lots of flexibility using barrel thickness and barrel spacing.

To those playing along...the right answer is right there in front of you, study all parts of the gun you see in the two pics of the 0000-frame Repro, put it all together...the correct answer is in the details and will become obvious to those that know their Repros!

The screw behind the trigger?

Nope, unfortunately that's incorrect. The screw(s) you see there is commonly found on all Parkers equipped with Parker single selective triggers.

The question remains "What's rare, odd and unusual with this 0000-frame .410 Repro?" I'm attaching the same two pics below so you don't have to go back and forth a page...

The single bead on the breech balls.

Unfortunately, that too; is incorrect. You're kind of skirting onto the right track though as the single bead at the breech balls is a commonly found feature on this model of 0000-frame Repros.

The forend metal does not have the raised round piece at top . Bobby

Are you referring to the forend tip that is not shown in either pic of this 0000-frame Repro?

No the little round pieces at each side at very top of iron that blends in with what i call rails of barrels and forend wood has shape of rails.Bobby

To me it looks like the bolsters do not stick out further than the sides of the frame . Where as on any original frame the bolsters protrude out further than even with the sides of the action body .