Ford Small Block camshaft thrust plate (front)

Ford Small Block camshaft thrust plate (backside)
Years ago I disassembled a 1982 Mustang GT 5.0 H.O. engine. Things never go as planned, so now I'm attempting to get back on track...I found 2 different thrust plates, but can't remember which one went with my engine. Both bolt on fine, and appear to be the same thickness. I don't understand the purpose of the different shapes. Can anyone identify why these two thrust plates are different shaped? Are they unique to a year? Does it really matter which one I use? Any information or history on the parts difference would be greatly appreciated!!! Thanks in advance....

 

From what I notice on the larger plate , there appear to be some openings that it covers. Other than that,you would need to measure the thickness to determine which is required if they are different thickness dimension.