Hi Gene,
Are you sure it doesn't say, "Plata?" Plata is Spanish for silver, and would denote a lower-grade silver (80% vs. 92.5%).
I've done a little digging since my earlier post, and am under the impression La Movediza stopped manufacturing in 1963. In fact, below is a picture of all the original La Modeviza die stamps, now in the personal collection of the nephew of the founder of La Modeviza.
Your stamp looks like it could be the 3rd from the left on the bottom row (though it's hard to tell for sure given the small image size).
BTW, La Modeviza was the very
first cutlery manufacturer established in Tandil. FWIW, Tandil (loosely speaking) is the Argentine-equivalent of Solingen, and it was in fact the WWII-era embargo of German exports that resulted in the formation of Industria Argentina's Tandil-based cutlery center.
Regards,
Chris