You have an M48. The M48A receivers were marked "M48A". As it happens, the later M48B model was also marked "M48A" (they did not change the stamping for that model).
The cracked hand guard is not unusual (as you already mentioned). The cause is usually that the screws for the (attached) clip were not tightened evenly at some point (this puts differential pressure on the hand guard wood). It can be repaired, if you wish. First, you must be sure that the wood is completely degreased. The BEST adhesives to use are either thin, high strength epoxy, or "gorilla glue". Gorilla glue is terrific for this, but the joint can be more difficult to hide, than with epoxy. Either will do the job well. In addition, adding some glass fibre cloth (glued to the inside of the hand guard) will reinforce the guard and help keep it from cracking in future. Finally, when re-installing the clip, be sure to tighten the screws carefully and evenly. Or, you could re-drill the screw holes and re-install the screws longitudinally, that is, in line with the barrel (instead of cross ways). That will eliminate the possibility of differential pressure against the thin wood.