Hi Bill and Glenn,
I think I may have the answer but can I clarify that you mean the Type IXs with the cut out foredeck (the schnelltauchback)? (since there was no IXC-42)
There were two IXCs and about 25 or so IXC/40s which had the cut out foredeck modification made to them. And a few IXDs too.
The cut out foredeck was first introduced in 1944 but this was well after the 105mm deck gun was ordered to be removed (order was on 27/04/43). So there should have been no boats with the 105mm deck gun and cut out foredeck.
There was at least one Type IXD (U 875) which had the cut out foredeck and a gun on the foredeck but this was a 37mm automatic rather than a 105mm. In any case this was a IXD which was very different from the IXC (and the Revell kits are IXCs not IXDs).
So, unless there was an exception for an unknown reason, it wouldn't be advisable to add a 105mm to the forthcoming Revell kit (the one with the cut out foredeck).
Cheers,
Dougie