Hi Siggy,
I believe there is a thread in the Vintage section about this particular guitar.
In my opinion the serial number is not original, however all other details seemed perfect for the first 1980 run of TE models.
As both 1979 and 1980 Tokai share the same plate spacing, it would be very easy to swap for an earlier plate.
This plate number would almost place the guitar a full year earlier than its release which seems illogical as there is no mention of TE models in either the 1979 Vol 2 catalogue or the 1980 Vol 3 catalogue (printed Nov 1979) and the official release date was March 1980.
I should also point out that serial number logs place other serial numbers surrounding 9005420 as being ST models only.
I am planning on visiting the Tokai Factory early next year to go through their archives and get some definite answers.
Peter Mac