Green’s Dictionary of Slang

underdubber n.

[dub v.1 ]

(UK prison) a turnkey.

[UK]Lex. Balatronicum.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.
[US]Matsell Vocabulum.
[Aus]Crowe Aus. Sl. Dict. 90: Under dubber, a gaol warder.