As part of a trade deal I recently obtained a very good condition Brno ZH 204 combo gun in 12G X 7x57R made in 1969. This is the same gun as the 304 but without the crudely etched "engraving" on the receiver. It came with a spare set of 12 gauge barrels. They are marked ZH 101, made in 1960. The spare barrels fit nearly perfectly but the action does not quite close by a millimeter or so (see photos).

My question is: does anyone have any experience fitting these spare shotgun barrel sets and if so what relieving needs to be done? There are various interior bearing surfaces and I'm wondering where a I may need to relieve a surface or two, proceeding slowly until a perfect fit is obtained.

Photos 1-8

1. Gun and spare set of barrels
2. Receiver, combo set and on the right, the spare 12 G. set
3. Spare barrel set on left
4. Interior of receiver
5. Spare shotgun barrels on left
6. With shotgun barrels fitted
7. With combo barrels fitted (flush)