Im not familar with the romanian rpk slings, the one i have is russian. But to me that buckle looks like its not the right one. is the romanian rpk sling wider than the standard slings? if so that buckle maybe is from a standard sling.
Romanians, like most countries, do not have an RPK specific sling. Early ones used the AK/AKM leather sling, then moved onto the typical nylon version still in use. Russians, at least, added the shoulder pad to most of theirs - mainly seen on RPK-74 slings - if I am not mistaken.
i have gotton 2 and 3loop paded slings from russia.different dates.have some more coming supposedly dated 1961 with nylon pads.that does not look like any slings i have got from russia.here is a pic of a leather 3 loop padded sling from russia.