The 3rd series of the 641087 block still only had 4 mount bolts for the bell housing.
The 641087 series block introduced under a test run in Jan 52 for 100 engines then finally officially started production in Feb 52. This block was erroneously labeled with casting # 641087 while it's blue print called for a new casting # 804380 but the production correction of the casting # at the engine foundry took place in May/Jun 52.