Green’s Dictionary of Slang

(US) a reliable or punctual person.

[1805		Mass. Spy or Worcester Gazette (MA) 20 Feb. 4/1: I mean to have my Sunday shirt / Wash’d and mended, early, / And I will be upon the spot, / As punctual as ‘Charley.’].
1835		D. Crockett Col. Crockett’s Tour to North and Down East 112: ‘Did not I tell you,’ said I, ‘that I would not vote on the appropriation bill, but when you came to anything else, I was “Charlie on the spot?”’.
1900		St Landry Clarion (Opelousas, LA) 16 June 3/3: Our friend J.K. Sandoz has already established a reputation as ready talker — no matter on what questyion, he’s Charlie-on-the-Spot’.
1905		St Landry Clarion (Opelousas, LA) 28 Oct. 2/4: The blaze was quickly extinguished. The fire boys, as usual, were ‘Charlie-on-the-spot’.
1942		Berrey & Van den Bark Amer. Thes. Sl.
1954		Dundee Courier 6 Dec. 6/2: Fortunately Fleming was ‘Charlie-on-the-spot’ a couple of times to cancel out Walsh and Mochan counters.