Green’s Dictionary of Slang

half-a-crown n.

[SE half-a-crown, in pre-decimal coinage 2s 6d (12½p)]

(bingo) the number 26.

[UK]T. McClenaghan Submariners I ii: tannoy (with everyone in unison): Two little ducks. Two and six. Half a crown.
[UK]P. Wright Cockney Dialect and Sl. 110: 26 = ’alf a crahn.
[US]J. Burkardt ‘The Bingo Code’ Wordplay 🌐 26: half a crown.