Serial numbers of the missiles thats all.
Missile XXXX might be made at a US facility and loaded onto HMS ____(insert one of the V-boats here), after which the US had no idea where it went. Then later on it came back for maintence and missile YYYYY was inserted into HMS _____ in its place. Once Missile XXXX is through the maintence process it ended up being slotted into the USS_____ (inseter one the USNs SSBNs in here) for a time before comming back to the US facility for maintence.
So what the Russians get is a number of serial numbers that tell them how many SLBMs are in the UKs hands at the time this set was handed over to them.
What they don't get is anything specific about that subtype of missile, or anything that might be different with the bus (which might even be seperate from the missile). Nor do they get anything on the UK RVs or the warheads contained in them.
Nor do they get to know how many missiles are in storage in the UK or onboard which SSBN, save for at best the one that came over to pick it up. Assuming the US has given that information, which considering we said "no" to giving over all the details, is unlikely.
Do remember we now remove the missiles when a boat is tied up, as it actualy extends the life of the missile being stored in a special facility. Also remember that the warheads are made in the UK though with some US sourced compoents, and are likely outside of any US serial sequence for their warheads.