In this context "transitional" means anything that was made in the few years before or after 1964 and assumes that styles and working methods would not have changed much over those years.
More specifically, for me at least, it means anything that does not easily fit into a "look and feel" of either a known Vasart (56 to 64) or Strathearn (64 to 80), but is clearly from one of those companies. In making a judgement, I compare the features to those I have seen in weights that are signed or labelled or have some other form of provenance.
Problem is, the more I see, the more I realise that some of the design or finishing clues can apply to both companies and possibly for a longer period before and after the 64 changeover date. So, in simpler terms it's a case of, "It could be either, but I ain't sure".
When exploring my web pages, please note that there are some things that need an update. Such as this weight
which, in the light of extra info, I believe is an early Perthshire Paperweights and not Strathearn. I must get around to those updates one day (hmmm, I seem to keep saying that