How to find ASIN on parent listing that is out of stock?

Hey guys,

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Long time lurker, first-time poster. I am looking for assistance regarding a parent listing whose child listing ASIN I cannot seem to find. For reference, here is the link. Unfortunately, the UPC code for the size 7.5 Black/Black/Anthracite (triple black) does not yield the size 7.5 ASIN in this case, but I am unsure of how to find the size 7.5 ASIN. I contacted seller support and they weren’t very helpful. Does anyone know how to navigate this? Thanks!