Sourcing caster insert/receivers for Marshall Cabs?

[strata2628]

Looking for receivers (threaded) for a '70's bottom cab....can't seem to find them for sale unless I buy the whole caster/assembly. Anyone? Thank you.

 

[Chester Nimitz]

https://www.ebay.com/itm/for-Marshall-4x ... Swax5YuMyP

These are about the best you can get but pricey.

Aluminum instead of plastic.