OEM for master cylinders???, 69 BB Cuda - looking for replacement gasket

The master cylinder in the Cuda works well, but the gasket allows fluid to leak and made my freshly re-painted MC look pretty bad in less than 2 years.
The gasket seems to fit well and the rubber is "soft / pliable", but I'm going to try and find a replacement gasket.

Some photo's of the numbers on the cylinder are below.
It looks like Dorman might make a range of gaskets.
Any other vendors you know of?


Numbers on the MC are
3699263 R2 on bottom spine
2427 or 2421 on mating flange.

master cylinder (1).JPG master cylinder (2).JPG