A friend of mine vaguely remembers in afghanistan a machinegun like a PKM where you could pop the feedtray and use a magaine like an AK mag, just bigger. He is not referring the Czech Vz52, which looks like a 2/3 scale bren and uses either belts or mags in 7.62x45, or the SG 43, which are the only belt or magazine similar gun I could think of.
Perhaps your friend is thinking of the Russian RP-46 which has a belt fed feeding module that could be added on but the weapon could also facilitate use of the pan magazine of the Russian DP-28:

I showed him the SG-43/DP-28 and RP-46 and Chinese version and he said no. I am baffled. It is possible he is misremembering, but he is sure about a BREN type magazine in 7.62x54. The sights he said would flip to the side for use with the box mag and that he practiced drills switching between the box and the belt.
M52/57 in 7.62x39 is the only mass produced MG like the one described that I'm aware of. Perhaps Czeckoslavkia also built some of the M59s with that feature.