This went low from a very knowledgeable collector and Forum member....Doug
https://www.ebay.com/itm/Bell-System-500U-Mushroom-Deskset-Telephone/173012538492
?hash=item284859d47c:g:9r0AAOSwEEBaI0ft
For those wondering what "quite low" was in a few years after eBay deletes the listing, here is a screen shot of the aution.