I believe the seven "Tin HALs" 2693-99, which were prewar HAL layout in a postwar SUB-style steel body, were some additional purchase with government compensation funds for wartime damage, and thus were somewhat separately identified for depreciation etc in the accounts. When the Gatwick Airport shuttle started in 1958 I seem to recall it was this little group that were assigned to the service, run on the rear of a Horsham semi-fast, detached at Gatwick so passengers and their luggage could detrain at leisure whilst the main train carried on, and reattached to the front of the next Up working for inward air passengers. The shunter on duty at Gatwick must have been pretty smart in their work.
I did notice that the quite common 8-car formations of 2-BIL units would have one HAL in them, and much later found a document which stated that the majority of the HAL units were treated as a pool with them and not separately diagrammed. However, it was almost always one of the inside units, as if they didn't like driving them. I noticed just the same on the Eastern Division, where again common 8-car HAP formations often had one of the Southern-design units included, again pretty much always in an inside position. It just seemed far too standard to be chance.
The Southern-design HAP units were strangely newer than the first mainstream BR-standard ones. They were on recycled 62' Southern underframes and used the old Southern body jigs at the works to fit these, whereas the BR units, with quite different bodies and cab ends, were on new 64' frames. The same was done with the two designs of 2-car EPB units. All these were 1950s construction for the big Eastern Division EPB changeover, associated with the Kent electrification, where the whole division was uprated to 750V and all the prewar units, which could not handle this, were cleared out from there in a big stock switcharound.