Hi Norton55,
I don't know whether or not my bike has its original tacho, but my Model 7 is fitted with a Smiths S467/3/L (sometimes found on the internet as "S4673L"), 0-120 mph. As your '52 model 7 will most likely be plunger framed, and my Domi is a 1950 and thus plunger framed, this tacho type might be the one...
PS: I believe this tacho is also fitted on Triumph and BSA twins of that era.
Good luck on you, Greetings from Rien (Netherlands)