The older Epoca is made for Ballograf's own "Original" refill. Fisher's PR series refills, without the G2 style adapter, will also fit, even though it's shorter.
Caran d'Ache's "Goliath" will also work but they aren't identical so the tip won't be in the exact same position.
The newer Rondo and Opus are made to take either an Original or a G2 style refill, which Ballograf actually also makes and calls "World" refill.
The Opus and Rondo will also accept some almost-G2 style refills, like a Fisher refill with adapter and Schneider's Gelion 39.
RotringThe Rotring Newton will work with Ballograf refills, but without a shim the tip won't be as far out as normally.
Their older model, 849, is made for their propietary "Goliath" refill and I haven't found any other which will work.
This is a bit surprising since the mechanism has the four prongs to engage a G2 refill, but it most likely didn't in 1969 and only have it now for parts commonality between models.
The Ecridor looks rather much like a 849 but it has a slightly longer barrel and is designed to accept G2 refills in addition to their own Goliath.
Ballograf's Original will also work, but due to different shapes the tip will not be in the exact same position.
But the almost-G2 refills I've tested, Schneider Gelion 39 and Fisher with adapter, does not fit.