There is a table showing this sequence on Skye's website, The MGB
Ericsmgs@aol.com had this to say:
>They did not have different speedos for the o/d cars. All cars I've seen
>- 74) used the 1280 series speedo. In 1975, the o/d cars went to 1000 tpm
>(turns per minute) due to the slight changes in the o/d gearing. I suspect
>the change in o/d gearing was due to the fact that the plastic gears on the
>1280 units wore out fairly quickly. But, 1975 - 1976 cars (non-o/d) used
>the 1280. And I have seen 1975 with 1280 and o/d. There was some weird
>stuff going on in 75 & 76. It all depends upon the o/d unit. Typically,
>black label units were 1280 and blue label units were 1000. From 1977 up,
>all cars (non-o/d & o/d) used 1000 tpm. Make sense?
>So many cars, so little time
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the red one with the silver bootlid.