There are approximately six variations of magazines for the Colt Model 1908 Vest Pocket .25. The earliest examples are full blued and have an unmarked floorplate. At approx. sn 105000 - 110000, the top of the magazine was hardened, thus creating the two-tone effect.
Type I full blued, unmarked pinned base, round top - 1908-1912 sn 1 forward
Type II two-tone, unmarked pinned base, round top - 1912-1915 sn 105000 forward
Type III two-tone, marked pinned base, round top - 1915-1920 sn 121000 forward
Type IV two-tone, marked cripmed base, round top - 1920-1922 sn 230000 forward
Type V two-tone, marked crimped base, square top with notch - 1922-1939 sn 306000 forward
Type VI all blue, marked criped base, square top with notch - 1939 to end sn 407000 to 409XXX, then 420001 to end of post-war production at 420705.
By rounded and square top, when viewing the side profile of the magazine, these shapes are apparent. The square top with notch was used to better engage the magazine safety disconnector (Tansley Device) that was introduced at approx. sn 141000.