The original from AC Thames Ditton described by Bryan Moseley is correct except that the pitch was 7/16inch. The chain and probably the chainwheels were Renold manufacture. The double-acting damper was much the same as a Holroyd (later owned by Renold) patent from the 1950s.
As noted by Bryan, life expectancy is considerable, dependent only on regular gearbox oil changes and a magnetic plug to prevent gear fines from clogging the damper pistons.
The original was a proper Triplex chain. My understanding is that Renold stopped manufacturing this and at some stage AC Scotland fitted 3 single row chains in lieu. This led to disasters - one row took the load and broke, following which the remaining rows continued to try and keep the machinery turning with the broken bits in the chaincase!
Following AC Scotland's demise, the Brian Angliss era AC Cars at Brooklands re-designed the chainwheels to accommodate a 3/8inch pitch Triplex chain. Note, the larger 1/2inch pitch chain could not be fitted into the chaincase. The damper was unchanged. The Company had some made as spares. These worked well although they are somewhat noisier and the noise is at a higher frequency (due to the shorter chain pitch).
These spares were all sold. The ACOC through its then spares subsidiary Acedes Spares Ltd had ten more sets made. These utilised all of the 3/8 pitch Renold chain recovered by Brian Eacott from AC's Brooklands stores and new chainwheels made to the same AC Brooklands drawings. These are a straight swap for the original setup, needing only a slight easing of the inside wall of the chaincase at the gearbox input end and very careful alignment of the two chainwheels.
A few sets are still available through Brian Gilbart-Smith. Please contact him for availability and prices.