In addition, a new kit-release has provided a far better foundation on which to build the master:
Click on Image for FULL RES
This is precisely as was calculated in advance: a 400-scale 3-inch bore should, at 1:550 scale to a 4.1-incher. (E.g.: [550/400] x3 = 4.1). That's just a 2.5% mismatch (oversized).
In any case - the Proof in the Pudding: virtually no difference at the tip is detectable against the above drawing: this gun is- and will enable a master far closer than the prior one (bottom, lighter plastic), adapted, as it was, from (and probably already itself undersized for) the Airfix 1:600 Hood.
In addition, the gun carriage already needed widening - to almost double its width - anyway, per my lone original reference pic. Now, doing so around this new, accurately-sized gun will be a joy to perform.
So, stand by for some progress on this little gem, at long lastů