First, welcome to the board.
I believe those were Spanish contract drums, actually meant for the MG1 type feed tray. In fact it is most likely marked '7.62x51 NATO' with a Spanish emblem which is similar to the Spanish Santa Barbara emblem (sword through a circle). If yours is marked differently please let us know as I'd like to see the stamp and other armory marks. Being metal they differ from the later MG3 plastic drums along with how they attach.
It's possible other countries used them, maybe even Germany prior to the MG3, but I have not seen any marked as such. I'm not sure of the makers (if they were contracted by Spain and made in Germany or made in Spain) or exact date of use, but I'd guess they came out in the late 50's when the MG1 came out.
They were intended to attach to the MG1 type FT and will not fit the MG3 FT. They were somewhat rare in the US for years (and expensive) then a stash of them came in about 10 years ago and many were listed on vendor sites as low as $25. Now they have again dried up and most of the time I see them they are about $100.
Related post with pics:viewtopic.php?t=13442