I believe these replace the traditional dynamo. It was an idea introduced on the XP64 stock, and as far as I can tell all new Mk2 stock used this system from new. Mk1s that received B4 bogies continued to use the original dynamo.
I think the uprated Mk1s B4 bogies still had the brackets, or at least the fittings for the bracket and dynamo proper. It's worth peering at photos.
B4/B5's on Mk1's with conventional dynamo's had a different (dropped) headstock to clear the dynamo belt - only the Mk2's and the Mk1 Postals (conversions and retrofitted with Alternators) and Mk2's with Stones Alternators used the headstock bracket. The biggest advantage (apart from the rather more up to date electrical system) was the ability to run at high speed without regularly throwing dynamo belts - a perennial problem with the underframe mounted dynamo and the single flat drive belt.