Mags for ICS 551. Can they be modded?

[0880yorkie]

I bought some MAG 100rnd mids (box of 2) for the SIg 551. Upon trying them in the magwell of my ICS 551 they do not fit. I have read that they can be modded to fit but how? They fit at the front it seems but won't locate under the mag release at the rear. Also the release button on the top of the MAG is at the rear where it is on the front on ICS mags and there seems to be a recessed area just forward of the hop on the rifle.