Cyril (right) as Chet Manley in "Live and Let Dine"
Chet Manley is possibly the full name of an intern Doctor Krieger killed in Season 2. Afterwards, Sterling Archer would use the name when he went undercover. In Stage Two, Archer lent the name to Cyril Figgis when the accountant was on the run for being wanted in Las Vegas, which were actually crimes committed by Archer using Cyril's name.
After Cyril becomes a field agent, he starts using the name when he is undercover, using it in Live and Let Dine and Sea Tunt: Part II.