Mesa can basically mod your MK III to whatever stripe you want, so it is not impossible that yours was upgraded from to a blue stripe. A gain mod is not really difficult, but is it on the lead mode or R2? You can add gain to both.
The easiest way to see what was done is to call Mesa CS and ask them if there was ever an RA/Return Authorization on your amp. If it was post 1990 that the work was done they would have the bill on file in there data base.
For example, one of my amps was pulled up under the serial number and I was told that the mid pot and the lead channel LDR was replaced as well as a full retube in 1989.
Boogiebabies, thanks for your reply! Actually, it's a channel 2 gain boost mod, not a lead channel mod. Don't have a photo of the rear panel with me at the moment (I'm in Japan, the amp is in L.A.), but the mod is very cool and I have never read here (or at other amp sites) about a gain boost mod. I read here alot about channel 2 vol mods.
As you look at the back of the chassis, there is a mini toggle modded to the underside between the far left EL34 and the left-side 6L6. Below the small-size reverb and presence pots is a matching 3rd pot (in an upside-down triangular layout) to adjust the amount of extra gain to channel two. Near the serial number is the added footswitch jack, factory stamped G.B. for gain boost. Interestingly, that G.B. is stamped "into" the black back of the chassis...and looks as if it was done when the amp was originally built (maybe a special order by the original owner). I say that because the black paint (or pwdercoating...whatever it is) is not chipped or retouched, like it might be if Mike B had embossed it into the chassis at a later date. But, I don't know.
Back in 2003 I was finishing school at Berkeley and drove over to Petaluma to have Mike go through the amp. He replaced some caps and threw me a few new tubes, and even said he remembered the amp (!). However, I failed to ask when the G.B. mod was done...and hadn't even considered the "blue stripe" question at that time. So, they have my servicing on file now, but I'll have to call Mesa CS and ask them if there was ever an RA/Return Authorization on my amp post 1990. Great advice! Thanks! And, interesting to know that there are other gain boost-modded Boogies in the world.