Greetings . . .
I think that one on ebay is an Italmeccanica/S.C.O.T. manifold (full back page ad from Speed Age July 1950):
although my research doesn't confirm a pop-off valve as part of this design. At any rate if it is a vintage S.C.O.T then it's a 216 manifold so it would need to be reamed out to fit a 235/261. Also, I am not sure how it got seperated from the blower it came with in kit form. These guys are still in business:
SCOT/Italmeccanica so it may be possible to order a new production roots type blower that will fit that manifold which is out on ebay. Of course money would be no object here . . .
As far as I can tell, these are quite rare units - hence the price.
regards,
stock49