I can't say what speedo you have, but if the odo is reading fast, then the speedo should read fast too. It may be that the head is toast. 1.05 is at the limit of what I'd call accurate, but if memory serves, its not to far out of line with what I've seen
I have an old Vicky brit catalog that I keep for reference and back when you could buy a new speedo, there was a different part # for the 1275 and there were 2 1500 tachs available.
1275 6-938
1500 6-942 and 943
-Michael