Thanks for sharing! A beautiful warm mellow tone that i've come to expect of such vintage instruments but which is seldom heard on modern pianos.
I've always wondered how much of that classic vintage tone is a function of the harp and the rim, since even with a new soundboard, that tone character seems to be preserved.
I don't think rebuilt Steinways are necessarily more mellow than modern Steinways. Although some rebuilders claim whatever good qualities a rebuilt Steinway has are partially due to the older plates, I don't see any reason why that would be true and have never heard any explanation even from those rebuilders.
I can't remember reading a single PW article that discussed plate design even though it seems like almost every other aspect of scale design and materials has been discussed ad infinitum.
I think the tone is probably much more related to the choice of hammers and strings, the soundboard construction, the voicing, and parts of the scale design other than the plate. I'm not aware of any major differences is the case design between new and older Steinways and if that could affect the tone.
But all the above is not based on any great knowledge of differences the plate or case might make, so I'm interested in those far more knowledgeable than me giving their thoughts.