Is this for a 12 gauge? Aguila makes a real short, like under 2" with either a slug or a mixed shot load. Made for the military and law enforcement but abailable to anyone. Lots cheaper that at that other link.
A lot of these guns were rechambered over the years--you might want to have a gunsmith check to see if it was already done. Of course a lot were also shot with 2 3/4" shells which is not the greatest idea.
When they changed the chamber length on the 16 ga. Model 12 in 1927 they also changed the radius on the bottom front part of the ejection port.
Even if you lengthen the chamber, shells will often hang up there when ejected unless that part of the port is altered.
Given the age of the gun I'd leave the chambers as is and the old girl unaltered. It's not like you're going to be shooting high volume clays with it so a few extra bucks for shells is not going to kill you. I've got one made in 1915 and I use it a couple of times every year bird hunting.