Well, I guess it had to happen sooner or later, particularly as I have a C5 (a 1981 model as far as I can tell) so here we go with the Celestron C5 Registry.
I've seen mention on the web that C8s were built at 3 or 5 times the rate of the C5 and certainly, C5 numbers have been a lot harder to come by than C8 numbers.
The general pattern seems to follow what one would expect from C8 numbers with the following sequences:
- very early tri-colour C5s have just a unit number, xx or xxx, eg 466 which was ordered in January 1972 but only delivered in October 1972 but was presumably given a serial number at order time or shortly thereafter
- by the second quarter of 1972, the C5 (still tri-colour) had moved onto the QxxxY and then QxxxxY scheme where Q = the quarter and Y the last digit of the year, eg 2 182 2 (made in the 2nd quarter of 1972) – this scheme appears to have continued up to the 1st quarter of 1978, eg 1 3658 8 which has motors dated 11/77
- then there seems to be an interim scheme of numbers used for the rest of 1978 and early 1979 – I can't tell yet, due to lack of numbers, whether these take the form of QxxxY or xxxxY, eg 4190 8
- then at some stage in 1979 Celestron changed to a non-date sequential unit scheme of the form 5xxxxx, eg 501610 with motors dated 04/80 and my scope 502612 with motors dated 4/81
However, unlike the C8s which kept the last scheme going from 8xxxxx into 9xxxxx, the C5s saw another change to a 5xxxx (one digit less) – I'm guessing (due to lack of numbers at the moment) that the 6digit 5xxxxx scheme kept going until the original C5 was withdrawn in 1986 (this was apparently to allow Celestron to boost C8 production at the time of growing Halley mania) and that when the C5 was reintroduced in December 1992 they started to use the 5 digit 5xxxx scheme eg 52126 for a white OTA advertised on Astromart and 53087 for a white C5+ in the CN classifieds, 54296 for a G5 reported to have been sold in July 1999, 58705 for a white spotter (also reported sold in 1999), etc.
However, the f6 telephoto lens version (with a black ota) seems to throw a bit of a spanner in the works, there was one advertised on Astromart with serial 4 059 5 (which compared with the normal f10 scopes of the era with numbers like 21436 5, 3 1506 5 and 1 1799 6 suggests that a different unit number sequence was used. Another (f10) ota, which may have been a spotter has number 4 350 9 and I originally thought that the 4 xxx Y was always indicative of the telephoto lens / spotter but I'm not sure now, as we also have 4 190 8 which appears to be a normal die cast orange tube C5.
Any thoughts? 
As ever, any further contributions, clarifications and corrections gratefully received. If known, it would be good to have the corrector number and any date on the motors and the original purchase date if known. Also, for the early numbers (ie prior to the change to the 5xxxxx sequence) please also give any spaces that appear in the serial number sequence if not obvious from a photo.
A link to the registry is here > Celestron C5 Registry
James
