OK, for those who even care, I strive to close open threads like this with some kind of resolution. So...three years later I finally purchased the plastic brake master cylinder cap from Pegasus Racing. It is a 2.5" plastic cap, Part No. 3573, and pleased to confirm it fits perfectly.
My whole reason for moving from the cheap metal to plastic (albeit not original) is b/c the metal caps would tend to rust really badly on the threads making a mess of the master cylinder -and occasionally making it hard to remove the cap. And I religiously bleed the brake and clutch fluid every 2 years.
Bob