There's one on ebay now. Bit of a roughy but has the S mark.

https://www.ebay.co.uk/itm/Vintage-Smiths-Chronometric-speedometer-S433m-for-XWD-BSA-M20-type/382304760031?hash=item590323b0df:g:8oQAAOSwPIhaKUnL

email (option): mfarnay@bigpond.net.au