Yep that's definitely it! Thank you all for helping identify it.
Looking at Simon Cornwell's street lighting encyclopedia, it does appear that the lantern was designed for SLI lamps, and therefore would have been converted like
Colchester's GEC Z9517 and
Colchester's Alpha Sixes. In fact in Simon Cornwell's write-up, he refers to the fact that the lantern could take two tubes, which may explain why the bowl is so deep!
I think there may be a chance to recover it. Although it's not on my contact's usual area, it is the adjacent area and he does occasionally get to see / work on other areas. But I'll certainly put in a request.
Knowing Essex County Council's policy, there must be a few more mystery relics around so I'll keep my eyes peeled! My contact even claimed the other day that there were some old fluorescents still going in Chelmsford, but I haven't found them yet! I'd be really surprised if this is true, as my contact himself hasn't seen them in real life, but he is aware of them from one of those lists the checkers use when they drive around looking for faulty lanterns.