Photo above shows the original (normal) Dunlop master cylinder. Notice location of port on side, hex head screws on top, and short rubber boot on the end.

This one came up on eBay in September 2010. Notice the port is located farther forward, top is attached with flat head screws, and the rubber bellows is not at all like the Twin Cam type part. So far no one seems to know the original application, or if this may be an aftermarket replacement part.

On 11/8/2018, John Gallagher [mgatwincam] wrote:
"I came into the brake and clutch business in 1969, and at that stage Dunlop were taken over by Lucas Girling (1966 I think). The latter could not supply any Dunlop spares only some old stock of repair kits.
"As listed there was an alternative offered in place of the Dunlop cylinder Girling part number 64068878. The Jaguar mk 2 also listed a conversion kit 64967012 and back in the day we sold plenty as the original Dunlop unit was no longer made. I have a lot of the old Dunlop manuals and can only see & remember the VBM 4022 being the only one listed for TC". - John

On 11/9/2018, Mark Hester [mgatwincam] wrote:
"The original Cap needs properly documenting, as it's unique to the Brake Master cylinder. Neil Burnside believes it may also be found on "Nash Metropolitan" but I've not seen proof of that yet. There are replica's available but they are unbranded and not 100% correct. If anyone has a better photo, please send them in. This one courtesy of John Barrett". - Mark