Complex sentence: It was also built up into the M75 Armored Personnel Carrier, one of the first enclosed box-shaped personnel carriers; that vehicle in turn would be the pattern for the M113 APC, which would become the most widely produced US armoured combat vehicle.
Split into simpler sentences: It was also built up into the M75 Armored Personnel Carrier, one of the first enclosed box-shaped personnel carriers. The M75 would be the pattern for the M113 APC which would become the most widely produced US armoured combat vehicle.