Above post is definitely correct.
However, the tenth position in the VIN gives the model year also. Yours is 'C', which would indicate 1982. However, that doesn't seem possible given the the 735i replaced the 733i for 1985. It would seem the oldest your car could be would be a 1985. I have an '86 528e, and the tenth position is 'G'.
'F' (85), 'G' (86), 'H' (87) 'J' (88), 'K' (89), 'L' (90), 'M' (91), 'N' (92), 'P' (93), 'R' (94), 'S' (95), 'T' (96), etc.
Are you positive that 'C' isn't a 'G'? If it is, then your E23 is a 1986, which makes more sense.