I'm 1955 they were all available from Holdsworth Cycling Aids.
Campag gran sport 75/
Simplex 543 79/6
Simplex juy51 43/9
Simplex TDF 27/6
Almost twice the price for the Juy51 over the Plain TDF and double again for the 543, they all seem to share the same materials and mechanisms.
Just for clarity here, the jockey wheel on the right in the pic above goes at the top.