Analysis of information sources in references of the Wikipedia article "Two six heave" in English language version.
The use of ex-sailors certainly left its mark on the London brigade. Firemen are hands who work in watches denoted by bell signals. There are no ropes, only lines. At one time the working cap of London was the peakless fore and aft cap of the seaman. When firemen pull or lift they count 'two, six, heave', not one, two, three. This, I have discovered, dates back to the days of muzzle loading cannon on the wooden warships. When the gun was fired it came inboard, after reloading it was pulled out by a block and tackle on each side operated by numbers two and six of the gun's crew. Thus the number one gave the order 'two, six, heave'.