[Senna4ever]

http://www.telegraph.co.uk/news/uknews/8015180/MI6-used-bodily-fluids-as-invisible-ink.html

[twitchyzero]

wrote in his diary that Mansfield Cumming, the first chief (or C) of the SIS was "making enquiries for invisible inks at the London University".
early April's Fool?

[eddoe]

FS: 10 Litres of invisible ink

[Razor Ramon HG]

early April's Fool?

http://en.wikipedia.org/wiki/Mansfield_Smith-Cumming

[StylinRed]

Maybe the term cumming (instead of come/came like it was back in the day) came from Mr. Cumming and his vats of semen for invisible ink