Ah I see. Okay now, I have a 1000mm belt on there currently but wasn't sure if it was correct.
The dual groove alt pulley I installed from an older MB alt is a wee bit bigger than the single groove that came off there (obviously something is amiss when I only have a single groove on there). The pulley I put on was right about 1/4" larger in diameter or so.
A 1037mm belt is just a hair over 1/8" longer than the 1000mm, so I wonder if the pulley I put on is from a much older motor with the longer belt!
Boy do I hate neglected vehicles due to bad PO's.