Yes on the two and three-car units. 4-car units didn't need them as they had guard's/luggage compartments at both ends.
The Black Triangle on a SR unit did not uniquely mean "there is a van at this end" on a 3car or 2car. It was far more complex than that.
In fact it meant .... that there is not neccesarily van at the opposite end (..... of a short train) and that the guards position is not at the front of the train.
"van this end" is a false fact put about by people over simpyfying and not understanding its origin, and others repeating it, and all forgetting that "trainspotters" for want of a better term know how units are formed but staff do not, well at least not the staff involved here.
When they introduced these markings it was for country rural stations where only one man might be on duty for parcels mail box flowers (Hamshire DEMUs) etc, and it included services worked by say 2Hap or 4Cep.
The rural steam trains that were replaced by Kent Coast electrfication and Hampshire Diesel were all per strict SR operating invariably formed kettle + 3set - and a 3set always had a brake van at each end, they were usually BSK+CK+BSK or some variation on the brake-nonbrake-brake formation.
Porters and postmen always knew with those steam trains where the vans were and where the guard was.
But porter Joe Bloggs on the platform cannot tell if an approaching train is a 4EPB or 2EPB or 2Hap, nor if it is a 4Cep or 4Cig. Postman Fred Smith won't know what a Cig or a Cep or a Cor is or was.
No black triangle on the front tells the person on the platform that while there
may (Cep, Cor, 4EPB, 6L) or
may not (Cig Vep 2Hap 3DEMU) be a van at the front, it does tell him the guard won't be there and must be further back down the train - important where mails are concerned. So don't move your barrow or truck to the front of the train.
If you think about this, it applies to all trains of D/EMU stock without anyone having knowledge of what unit types the train is formed of.
Black triangle yes does impart the info there is a van at this end - but that is neither the origin nor the complete reason for it.
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Nick