looking through the catalogs that cover the model 1908 i find that the chambering in the 12 was listed as 2 1/2 inches until 1920. 1921 and after it was listed as 2 3/4 inches. any model 1908 that is suppected on being manufactured before 1921 should have the chambers checked because both iver johnson and H&R offered rechambering of their earlier gun.
Iver Johnson's Arms & Cycle Works 1871-1993
H&R Arms Company 1871-1986 (due spring 2010)
available from www.gunshowbooks.com