what sort of service protective device is this?

well, inside  the Lucy box will be a load of 100A fuse holders, one per meter, with a 100A or less fuse in each.

Depending on the model there may also be DNO incoming fuses at 315 or 620 per phase under the smaller of the 2 doors.

These sort of things


Actually, it is a lot neater than one I saw recently where there is a horrific confustication of Henley blocks and short lengths of tail to achieve a similar effect, but in open air, with 21 flats and a 'landlord' meter split out across 3 phases, each with a 60A switch fuse as well. In that case though the incomer was 3 phase 100A, so some quite fierce diversity assumed, or more likely it just grew like topsy and has never been designed formally.

well, inside  the Lucy box will be a load of 100A fuse holders, one per meter, with a 100A or less fuse in each.

Depending on the model there may also be DNO incoming fuses at 315 or 620 per phase under the smaller of the 2 doors.

These sort of things


Actually, it is a lot neater than one I saw recently where there is a horrific confustication of Henley blocks and short lengths of tail to achieve a similar effect, but in open air, with 21 flats and a 'landlord' meter split out across 3 phases, each with a 60A switch fuse as well. In that case though the incomer was 3 phase 100A, so some quite fierce diversity assumed, or more likely it just grew like topsy and has never been designed formally.