B for Beckton, Bank or Bombardier?

Really? Why, then, were the original units, built by Linke-Hoffman-Busch, and the later ones built by BREL at York for the Bank extension, known as P86 and P89 stock?

I will certainly be very shocked if that turns out to be true.

I've always assumed that B denoted Beckton, however on the face of it there's a slight lack of logic to this as Beckton (both extension and depot) didn't open until some time after the B90 stock entered service. With that in mind one wonders if B could, at least to some extent, refer to Bank? Although the P89 stock did run to Bank, it was always the case that the B stock and Bank went together.

The only units built by BREL were the P89s.

Although BREL was later bought up by Bombardier, the later units were not built in the UK.

The B90s may have been ready before Beckton depot but they were not maintained at Poplar - there was no space for them. I recall them being stored in a long line on the not-yet-open viaduct of the extension

Was it not the B92 stock which was stored? There was definitely B90 stock in service quite some time before Beckton opened. I seem to remember the signalling fitted was initially different - either the B92s didn’t have the original signalling fitted or the B90s didn’t have the Seltrac fitted, I forget which way round it was. In those days the service was generally B90 on Bank to Island Gardens, P86 on Tower Gateway to Island Gardens and P89 on Canary Wharf to Stratford. The Official Handbook from the time may well elaborate more.

I think it sensible to assume that the intended eventual destination for B stock was Beckton as the previous destination was P (Poplar). Either way, the manufacturer designation theory doesn't seem to hold much water due to the previous P stock designation.

I thought to look at copies of Underground News, the London Underground Railway Society journal. Those from 1986 and 1987 can be consulted on their website.
I looked at references to the DLR rolling stock. None of those references gave a name to the rolling stock, so P86 seems not to have been a designation known in 1986.
This gets curiouser and curiouser!

Edited addition: not even P was used, just the unit numbers.