I don't have a clue what a model 55 is but but if it's a marlin or one of the major gun makers give them a call you got nothing to lose. Have you try numrich gun parts they have a magazine for a 55 model in 12 gauge maybe it will fit or you can make it fit.
Numrich Gun Parts Corp.
MARLIN 50 BOLT ACTION RIFLE, 512, 55
Magazine, 12 Ga., 2 Shot, Replacement
19 958930A $27.00