The third plate saw use in 1952, and I do not find any of these varieties on the 5c. I find all of these on the 10c Rivadavia Brown and the 15c Martin Guemes, both on three papers: 1L (ondulating rays of 1951), 2D (straight rays diffused from 1943), and NGR (unwatermarked grid from 1945).
I think that there are two possibilities:
1: Printed stamps from when these papers were first issued were in stock and overprinted in 1952.
2: Made to order batches of the base stamp were overprinted with rolls of these papers in stock in 1952, and subsequently overprinted.
Here are the varieties;
Rounded L in the form of a boot
L with lengthened leg
L with shortened leg
L deformed and thin
Larger blocks would answer lots of questions. Unfortunately, all I have are blocks of four.