help,,chasis number missing digit?

[jack1]

hi folks as mentioned previous i;ve misplaced/lost the key to my 1983 coupe gt now audi seem to think im missing a letter from my chasis number on the logbook,,,if any one has a coupe gt of the same year or in around the year could they check it for me please mine is , ...... 85da ...... they seem to think there is a letter missing before the d or just after it??? and there should be 17 in total there is 6 numbers after the a ,,,thanks