Retainer for front crankshaft seal. Is this a narrow deck engine? Engines that don't use these retainer plates usually have small groove in the case where the seal goes and they use a 1 piece seal with a little ridge around the perimeter that fits in the groove. My 320 has the groove and does not have tapped bolt holes on the very front of the crankcase.
I think you are right, it must be the front seal retainer. I believe this is a 1960's wide-deck engine. It does seem to have the holes in front.
Neither of my parts catalogs (2011 or 1984) show this retainer, but I found it in a 1970 version of PC103. It also gave me the hardware callouts, as well as the correct front seal part number (76940, the seal without the little lip). The older PC listed both types of front seal, and said to use this one if your case has the bolt holes in the front. Awesome!