Green’s Dictionary of Slang

doonup n.

also dunop
[backsl.]

£1 sterling.

[UK]Chambers Edin. Jrnl XVI 192: ‘Eno dunop and a flah,’ says a new bidder. ‘A noc more,’ adds a competitor; and the lot is his for L.1, 15s.
[UK]Hotten Dict. of Modern Sl. etc. 124: DUNOP, a pound.
[UK]Hotten Sl. Dict. [as cit. 1859].
[UK]Sl. Dict. [as cit. 1859].
[UK]Nottingham Eve. Post 9 Oct. 5/5: Back-slang [...] as in ‘dunop’ meaning a pound.
[US]J. Burkardt ‘Back Sl.’ Wordplay [Internet] doonups: pounds.