In Reserved Material, on SharePoint, in the list settings, I would make Tracmor number a lookup column that looks up the Tracmor number from the Stock list. That way you can only select valid numbers. After you do that, walk through the data connections in the form again to update them. Then, your comparison for the dropdown where you wish to only display stock(N°Tracmor) would be able to compare on the ID.
Hilary Stoupa
