Now I understand about your question in the Grant thread.
Until someone sends you some better pics, have a look at the rear view of the Italian Sherman in the link
http://www.missing-lynx.com/library/usa/sherman/m4a176/127-2.jpgTo stow the .50cal, the barrel is unscrewed and the pintle placed in the mount on the turret rear and clamped (the bulge is the turret vent fan cover). The end of the cooling jacket of the .50cal is fastened into the clamp on the left of the picture (the circular object fitted on a frame, in front of the window on the back wall). The barrel is fitted into the spring clips on the angle irons (these are just visible as small square fittings, one on each angle iron).
I hope this make some sort of sense.
If you're looking for the clamp, then most PE sets have them, or if you want to go plastic, the Dragon Firefly has one as a spare part (it includes a sprue from the M26, but only the .50cal is used from that).