[GreenKeys] Model 15 serial numbers

[Jim Haynes]

Well if we had the Teletype invoices that Ed mentions we could figure it
out because I think at least for non-military customers they would invoice
it as a BB-something, BP something, BK something if there was a keyboard,
BC or maybe BPC for the cover and XT something for the table.  Military
customers might have got TG-7B or the like as a complete set.  Which might
mean a different series of serial numbers, tho I guess the things inside
would be individually serial numbered.