I knopw what you mean for warships in particular. When I was talking about swapping masts on merchies it was because with all the thousands of merchies they built over 1000 tons, the biggest single "class" was only a couple dozen ships or so. The bulk of the rest were 1s, 2s, or 3s---and even those were different from each other.
So a "generic" MFM, counter stern based on a REAL ship with a couple alternate masts, etc might well allow making
dozens of different ships.
For example:
The 2d ship there, there are 6, and 22 more that are "similar". In addition, there are more MFMs that are close enough that she'd be a reasonable "fill-in" sionce there is no way we'll ever have 1000 classes of mid sized jap merchies
