A nice-ish looking early gearbox on Ebay in Australia ... bidding is at $200 AUS
http://www.ebay.com/itm/BSA-M20-GEARBOX-/191935071028?hash=item2cb038c334:g:BpQAAOSwARZXnXsL&vxp=mtr
Oddly this box isn't listed on Ebay Australia but listed on US Ebay, postage from Australia. I imagine the seller thinks that he'll get a higher price there.
Someone has had a go at that main shaft so buyer beware.
Mike Riley has a partner in the USA that does all of the evilbay listings.
Mike does not use any computer based sales platform himself.
I don't think he even uses a computer , the bugger never seems to answer emails but always phones back when he is in OZ.
He really does not do much in the way of used parts except for Gold Stars which he specializes .
He is one of the partners who bought out Cycle Hub and Accessory Mart.