I got some 4.53 from a Czech lad last year who had some connection with JSB and he assured me that the head sizes were just marketing, I have measured plenty and I must agree.
Still I like to use 4.53 in my springer
Pavel was at WHFTA, could have given him the 3rd degree but I suspect we would have got the party line, cracking bloke though.
My uneducated feeling is that as JSB have soared in popularity wordlwide, the dies have changed to suit faster and bigger production. To my eyes, JSB insides now look like the 7.9 Prems, flat and shallow.
The new pellets I have measured seem shorter, like these Premium Exacts. My old 2006 JSBs are longer with pointy insides, and I think seem more stable.
In the winds these days WTF knows