What does M2029 mean on a marlin 22 cal model 56?

My guess is that it is the serial number. The model 56 was the first of the Marlin "Levermatic" line of guns that featured a clip magazine and a very short throw lever that allowed the shooter to cycle rounds through the gun more quickly than a standard lever action rifle. From it came the Model 57 and 57M which were tubular magazines of the same gun and the model 62 which were .256 win. mag and .30 carbine versions of the "Levermatic".