Complex sentence: An early fix employed a roughly horseshoe-shaped steel bracket attached around the rearmost part of the receiver to reinforce where these plates dovetailed into the side plates; a later riveted right-angled steel "stirrups" to the bottom and side plates.
Split into simpler sentences: An early fix was to attach a roughly horseshoe-shaped steel bracket around the rearmost part of the receiver. A later fix was to rivet "stirrups" (right-angled steel pieces) to the bottom and side plates.