The jacks are the only similar (but not identical) parts shared between the StuG and the Hummel (for X and Y purposes, anyhow). There haven't been any reports of StuG parts on the Y, but ya never know.
The Mustang yields the landing gear half used on the Y's neck. The Hurricane has different gear. To my knowledge that's the only contribution of the Mustang to the Y, but I'd be happy to be proven wrong. (The oil pan cover is 99.99999% identical to the Hurricane's, though, so you needn't buy two Hurricanes or cast that part to complete the set on the Y.)
It has been reported that parts #5 & #5A of the Harrow are on the Y, but exactly where hasn't been clarified. I don't have the kit handy at the moment to take a look at the parts, and I haven't gotten around to searching reference for those particular parts yet.
This is what was previously reported about the Centurion:
</SPAN><TABLE BORDER=0 ALIGN=CENTER WIDTH=85%><TR><TD CLASS=$row_color>Quote:<HR></TD></TR><TR><TD CLASS=$row_color>control panel part used on the side detailing of the ship, just behind the R2 unit; it is a rectangular part that has lots of little details on it. the 8 Rad chassis is in front of it.</TD></TR><TR><TD><HR></TD></TR></TABLE><SPAN CLASS=$row_color>
I don't have a Centurion yet so I can't elaborate on that.