Mark I's, II's, III's and the almight IV
Living (and buying a good amount of amps) in Northern California I can say there are all kinds of weird Boogies floating around the Bay Area. Early prototypes, no serials, employee amps, strange customs, etc. Also I hate to say it, but a lot of people who were early Boogie customers/employees are starting to die. I've bought a lot of amps from people's next of kin so the history is often lost or unclear.
MESA/Boogie Timeline: http://mesa.umig.org/boogie-timeline
Interesting! The R2 channel was developed from an original ++ mod which added in a switchable capacitor on the V2A cathode which does what you describe. However, the 'R2' channel in the early black stripe does something similar and the R2 channel developed over time with massive changes between the stripe versions.It's seems to be a mid boost of some sort.
So depending on which R2 your used to, it's either a stock black stripe R2 or it has a switchable ++ boost. Who knows! Either way it probably sounds killer!