I didn't believe it myself when I saw what was there -- thought that those large chrome dome heads through the outer of the "B" post were fixing remnants of semaphore indicators from Mk VII days. But yes, that's what they'd done on that '59MY Mk IX. My '61MY's still in store, waiting completion. As I type this I've just agreed with another shop completion for <drum roll> April of next year

And am searching for a pair of non-frayed Brytax lap and diagionals of the period.
I'll be seeing it at collection next week, so will have a closer look
Just had a look through the last-published Spare Parts Catalogue (Feb. 1962 £1.5s.0d) and seat belts not listed even in the optional extras' list. Although the diagram of the "B" post does have two holes in its length approximating to where those large chrome dome head bolts are in my pic of my '59MY. Jochim Lausmann had collected far more info than me. But I don't think he's still with us...
I've also looked through all the advertisments I've been able to cadge and there's nothing there except what appears to be those "B" post bolt heads showing in a side view in Marston's advert (
Motor published 1958 top pix)