I don't have an EP 120 on hand for a double action, but tried for fit a few other well made holsters with a Sec 6, K frame, L frame & GP 100.
Security sixes "almost always" share a holster size with a K-frame. The L Smith & the GP share a slightly larger holster. My GP & L-frame don't have the barrel under lug & they fit a lot of my K frame holsters. Doubt they would if the underlugs were present.
Not huge differences in K to L holster fit IMO. When you put a K in an N-frame holster is when the rattle starts. Heavily boned or tightly fitted security holsters are a lot more model specific.
Hope some, any of this helps.
One more thought. The newer 4" K, L, & GP's are now usually made with 4 1/8 - 4 1/4" barrels to accommodate a silly Canadian law, so the closed toe on an older EP 120 might cause a problem.