The Fender Showman amp was essentially a head version of the Twin Reverb, without the combo's reverb. Factory cabinets came in a 112, 115 and 215 versions--the latter known as the "Dual" Showman. Speakers were a JBL D-120 or D-130s. (Later on, a 15" bass speaker--the D-140, was offered as an option so bassists could use the powerful Showman amp.)
In the mid-'60s, these cabinets were small, and fairly compact--and IMHO, they are some of the best speaker cabs ever built. Also note the small Bassman and Bandmaster 212 cabinets; again, excellent sounding speakers. Fender cabinets had fittings for thumb screws on the top surface, and heads came with hardware fittings underneath the head cabinet--the two pieces could be mated for a "Piggy-back" configuration. (Fender held a patent on this device. Another patented feature were the "Tilt-Back Legs, allowing the amp and cabinet to be tilted back to roughly a 45-degree angle.)
Of course, Leo Fender, facing uncertainty over some health issues, sold out to CBS in 1965. Shortly after the sale, these "Black Face" amps were redesigned to become the "Silver Face" amps. I think it was about 1967, when the amp cabinets were redesigned as well, becoming much larger and taller. Rather than a horizontal 212 or 215, the speakers were vertically oriented, probably done more as a marketing move to compete with the newest British invention--the Marshall stack. These larger speakers are not as generally well-regarded due to their over-sized bulk--though as a rule, they do sound pretty good.
Over time, the Showman's petite 112 and 115 JBL enclosures were dropped from the catalog, and the Showman officially becomes the "Dual Showman" and is badged as such. The Bandmaster and Dual Showman soon receive larger head cabinets as well, to accomodate a reverb tank. Silver-faced or not, these are ferocious amps designed to play clean tones at loud volumes. The now Dual Showman Reverb is simply a renamed Twin Reverb.
It is not unusual to see these amps these days heavily modified from their original condition, with "black-facing", effects loops, channel switching, 1/2 power switches, and--installing the head into a combo cabinet. Power specs in these beasts range from 85 watts to 135, so I doubt a 2-8" cab could produce power for very long. It more likely has heavy-duty 10s--perhaps an ode to the great Music Man 210 HD-130s of the Seventies.
One of the best of these head conversions that I have ever seen was a 410 Dual Showman Reverb version, using EV-10s. It looked totally factory,...a 135 watt "Super Mega Colossal Reverb", if you will. The guy who owned it was a country Tele picker, and he wanted clean headroom and lots of it! I think it must have weighed in at nealy 150 lbs., too! And that amp has always made me wonder why Fender didn't make a 410, 100-watt version of the Super Reverb. The closest they came was the Quad Reverb (known for being Albert Collins' fave); and the Super Six Reverb. These amps are all based on the Twin, with different speaker configurations.
So, your find isn't a "factory" piece, but if the amp is up to snuff, it should be very clean, very loud....
...and GLORIUS!
Bill