this is all off the top of my head which is highly unreliable
I believe the HO52 is a 5200lb axle and HO72 is 7200lb..
and I believe the HO72 brakes are bigger, wider if I recall correctly..
and I also believe the third members were the same, only difference was the side load adjustment on the HO72, but later HO52 had them as well..
factory installed no spin has the small oval tag on the pinion cover, top pass side bolt.. nothing on the tag but its presence is the tell..
I don't have facebook, where's that third member located? thanks!