This Type 24 appears not to be listed in the DOB or EDOB databases.
Update 21/6/2020 it now appears as PILLBOX (TYPE FW3/24): e41998
The photos in the gallery below don’t appear to have aged very well and quite a lot of work has gone into recovering as much detail as possible.
This pillbox may still exist – anyone want to volunteer to see if it does?
Update 29/5/2020
Whilst investigating the visible above-ground characteristics of the cut-and-cover Kidbrooke tunnel on the Bexleyheath railway line, I came across what I now realise is a pillbox. It’s position corresponds precisely with the blue ring on your location map, and it can be easily accessed via Streetfield Mews on the Cator Estate. The tunnel position is easily spotted in the form of a long straight grassy bank that junctions with the road. The pillbox is located at the end of this bank, in a shallow wooded area above the tunnel portal.
Richard Stockall .
Pics by Richard Stockall.