It looks like a double column which reduces to single feed. What is the cartridge OA length capacity? Photos are nice, but without some reference measurement, we are stumbling in the dark.
Possibilites are CSRG 15/24 Belgian (7,65x53) or Polish/Greek ( CSRG 15/27) 7,9x57; Definitely NOT Japanese (6,5 or7,7) as the Mouth of these is totally different, and L-R feed.
Other MGs ??? Madsens are single column...could be a DIY M24/29 MAC LMG Mag???
Doc AV
