I contacted that ebay vendor to see if they were certain their webbing satisfied seatbelt requirements, and this was their response:

"Hi. We have been selling seatbelt webbing since the year 2000 and we buy it from a place that deals with just seatbelt webbing. I never had a complaint about it. If it wasn't seatbelt webbing I would state that, If you check my listings you can see the different webbing we do sell. Because we sell regular nylon, cotton and elastic webbing.
Thank You"

They then signed their name and provided their phone number. Clearly, they could say anything, but it seemed to me like an honest response.

Still a little confused about the routing of the webbing, but oh well.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).