"I hardly can say that -504 and 3M-M has many common in their design."
You're right of course, they have different powerplants and apart from several bolts
and nuts you probabably wouldn't have found identical components, but ...
both were derivatives from landbased aircraft, designed in the same way, by deepening
the fuselage in quite an extreme way, as both basic aircraft had very slim fuselages,
and fitting them with a convenional flying boat planing bottom. There were few landbased
variants of flyingboats planned (e.g. of the Saro Princess), but for the other way round,
there are even fewer examples, The only other I can remember, was the Convair design
for a flying boat derivative of the Comet .But that was an aerodynamically surprisingly
clean design, something I really cannot say of the Tu-504 and the 3M-M.
But ok, to compare a heavy prop bomber and a jet bomber may have meant over stretching
ones phantasy, although both were designed as combat aircraft, with the same simple and
probably unrealistic idea in mind.