Green’s Dictionary of Slang

(N.Z. prison) the portable, silver-painted, steel gallows, moved and erected as and when required.

1980		D.F. MacKenzie While We Have Prisons 44: At this time it was decided to have a movable scaffold capable of being transported to wherever required [...] A later version of this contraption became known as the Meccano Set – a term used in official telegrams ‘Meccano Set arrived safely and erected’ [DNZE].
1998		N.Z. Herald 25 Nov. 🌐 ‘The Meccano Set’, was used to hang 20 people. The last was convicted poisoner Walter Bolton in 1957.
2005		(ref. to 1920) www.nzcrime.com 🌐 Euphemistically christened the ‘Meccano Set’, the silver painted scaffold made its debut at Mt Eden Prison on Christmas week of 1920. Its first client was Samuel John Thorne who had been convicted of the murder of his ex-employer some three months previously.