DD,
The action is a takedown Model 6 and I was able to use the original breach block for both rimfire chamberings without any modification. Because there is a small difference in the head diameter of the .17 HMR and the .22LR there could have been an issue with the point of striker impact on the .17 HMR rim creating a problem with ignition. That didn't turn out to be a problem.

The centerfire breach block started as a rimfire and its position in the closed position was at the same height as the original rimfire block so the geometry of the block face to the breach end of the barrel was essentially the same. The change over to centerfire was pretty straight forward.

Parts for miniature Martinis are no longer easy to find so having another action group wasn't considered. When changing between rimfire and centerfire only the extractor and the block need to be swapped.

There is only one takedown screw for the four barrels and that was a bit more of a challenge. Morris used EDM to cut the tapers at the bottom of each barrel's threads and this works quite well.

Hope this answers your questions.