The engine displacement classes in Japan [& elsewhere] during the 70's were 1300cc & 1600cc + larger. Anything more than 1300cc in an A series back in the 70's put competitors at a serious disadvantage because they were in the 1600cc class, but the 1200 blocks, & later, the A12 blocks would bore out to these dimensions & made these engines very competitive.
The A15 didn't come along till much later & even then, the 79mm bore size & the 1608cc capacity that this bore produces is a relatively recent development. Nissan had perfectly serviceable L series engines for these larger capacity classes, so 1600cc A series engine were never a priority, probably never even given a second thought.
So to answer your question, these pistons were intended for 1200 & later A12 engines that competed in various events in the 1300cc classes.
As for the rods, I thought for a moment that they might have been the factory Titanium rods, but no, they look like regular rods, [see picture of Titanium rod below] so these H beam rods are from an aftermarket manufacturer, but I have no idea who.
I remember pricing the Titanium rods, new, through Datsunsport in Melbourne back in the early 80's. At that time the quoted price was something like $990, .............................. each.

With appologies to Feral whose image I have shamelessly pirated.
For more info on the amazing engine that this rod belongs to,
Click Here