These are head extensions that allow to extend the masts in order to install fittings in overhead power lines.
The standard elements are:

Use in standard masts
EXT. C2000 – 1,21200510C-1000 y C-2000
EXT. C2000 – 1,81800510C-1000 y C-2000
EXT. C4500 – 1,21200510C-3000 y C-4500
EXT. C4500 – 1,81800510C-3000 y C-4500
EXT. C9000 – 1,21200510C-7000 y C-9000
EXT. C9000 – 1,81800510C-7000 y C-9000
EXT. C: Extensions for support kind C.
2000/4500/9000: Nominal effort of the support to which it can be allocated in daN.
1,2/,18: Height of extension in m
Longer lengths can be produced on request.