Most .410's of that vintage were made with 2 " chambers. The 2 1/2" cartridge did not appear until around 1913. The little 2 inch ones were often called collector's guns because they were intended for taxidermy specimen collectors. I have 2; a single and a double, that have had the chambers lengthened and then passed re-proof. Not that many were made and are usually of top quality. Lagopus.....