The speedo drive unit on the rear wheel is the 1st place I'd be looking.
A chronometric mph speedo requires a 2:1 ratio.
Pretty much all later speedos use something that spins the cable slower/faster.
I'd guess you have something later.
I don't recognise that speedo number at all in respect of chronometric speedos.
Smiths had a convoluted system of numbering, which takes some deciphering.
However, that 1050 in the speedo number could mean its something quite unusual,
Its related to rpms, and I can't readily find any reference to it.
Could it be km/hr, with a mph face ???
The parts list
for my Ajay lists all the chrono speedos that could have been fitted from new that year,
do you have similar ?
So, what is written on your speedo drive unit ?