A 1936 mfgr should be 2 3/4" chamber and should be marked on the bbl.
Change over from 2 1/2" to 2 3/4" came about around 1926
The 2 1/2" chambered guns were not marked as to chamber length.
The very first of the 2 3/4" chambered guns had the chamber length marked on the bottom of the recv'r right underneath the Ser#.
That only lasted for a short time until the supply of early unmarked bbl's were used up.
Make sure the Recv'r(Frame) and the BBl assembly ser#'s match. Look closely as many have been 'forced matched' when it comes to ribbed bbl guns. Look for the bbl assembly number having been removed and the new matching number stamped into place.
The correct font usually isn't quite correct and the bbl assm ser# will be stamped lightly in an effort to avoid crushing the thin bbl extension ring.
The ring has to have been reblued after these efforts. Some are professional,,others are a quicky cold blue attempt.
Proof mark on the bbl should be on the left side of the bbl at the breech just off of the Raised Rib.
The matching proof on the Frame should also be off to the left side of center at the top front of the frame.
Both should appear to have been stamped 'Thru the Bluing'
NOT blued over the stamped image. Definetely not polished over.
A decent buy on a true 90% Mod12 pre-war 20ga w/raised rib in excl mechanical and bore condition IMO should be betw $700 and $1000
The difference comes in when intrepretations of what 90% means.
and...What the bbl length of 28" means to you the buyer ..Good or not so Good.
Even if you like a 28",,you can say you were shopping for a 30" ..which is very much the popular want these days in a small bore.
Same with the Choke.
(Check the choke with a gauge and see if it is what the factory marking states it is.
If it has been honed out,,then the price lowers as it's non-factory,,,
The price lowers even if the alteration is to your liking. Time to kick tires.)