These were fitted to the standard bantam D1 as an extra, you could remove the single seat and attach this in the front, and the back would rest on the rear carrier, hence the clip type brackets at the back of the seat that would clip under the carrier, although looks very rusty, it is good and solid, the foam has disintegrated over the years, but the mounting brackets, and base, are very solid, this is the same seat that is also fitted to the BSA plunger C11.
Please call for other BSA spares not listed on the site, we have many in stock.