I had the same problem here. As explained by Sandie and antp, the Mk field must be filled with digits only, not with I / II nor with a / b...
The script translates it then in roman numbers, or, if I'm not mistaken, in letters for Opel.
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If Ddey65 is Ok with it, could a forum moderator rename that thread How to fill the Mk field when adding a new vehicle or something like that? This problem seems to occur regularly, so it could be helpul to find it more easily in the forum?
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Edit: Oops, I should have refreshed the page before posting. You were once again faster than me, Sandie