Hi Pearl-Drum,
It's not a serotta frame. It is as you speculated, something of a 'commemorative' bike. It's most likely of japanese manufacture (the chinese weren't exporting bikes to the US back then). It also isn't a very good bike as far as bikes made for 'cycling enthusiasts' are concerned, though it was probably the best bike murray had to offer at the time. It was probably originally bought in a department store like Kmart. I can almost guarantee it didn't come from a bicycle shop. Here are the dead give-aways:
The big reflectors on the front and rear: Racing bikes - even low-models - don't have them and never did. Even in todays super-litigious and safety conscious society, bikes purposed as racers at the most have wheel reflectors, and those are even banned in beginner races.
The shifters are clamped on with a clamp around the tube. Bikes of good quality (even touring bikes) back then had small "braze on" studs on the tube where yours has the clamp.
The wheel clearance/ geometry - If you look at the two serotta examples, you'll see how the clearance between the wheels and the frames is very tight. On your bike you can almost slide your balled-up fist between the frame and the wheels.
The components - while the manufacturers you listed _do_ ( and did) make some decent components, the models on your bike are not those.
26 pounds was never light - except before world war one.
Racing frames - even low-end - ones do _not_ have the little eyelets on the frame down by the wheel hubs. These are for luggage racks (aka 'panniers').
The _good_ news is that the frame is reasonably well made with lugged tubes and forged dropouts. (the 'drop outs' are the thick pieces of metal where the wheels are bolted to the frame. On **** frames they are folded and stamped sheet metal. On decent to high-end frames they are forged, and many high-end frames also use CNC machined dropouts). For a mass produced frame, that is pretty good (though any framebuilder would still be embarrassed to have his name on something like that, but still you wouldn't get much better from a factory). Another reason I think it's the best frame murray offered was that the shifters are down on the "down tube", not clamped up on the handlebar stem like most department store bikes.
That said, I wouldn't put any money into restoring it as original, but it would make a good single-speed or fixed gear conversion bike. If you take it to a local shop the conversion can be made relatively cheaply. Basically, you would remove the derailleurs, the big chainring, the free-wheel, and the brakes (sorry, those brakes are total ****).
The shop would put some decent modern brakes on (they're actually pretty cheap), and a single cog freewheel or fixed-gear on the back. You might also want to replace the saddle. Realistically, you can get this done for less than 500 bucks including labor, and probably less than $250 if you take it to someone you know who knows how to do such things, or do it yourself. It's actually quite easy. Just an FYI - fixed-gear/single speeds are _quite_ trendy these days, and small bike shops _love_ doing that kind of stuff.
go here for details http://www.sheldonbrown.com/deakins/how-to-fixed-conversion.html
The serotta bikes you showed were built by Ben Serotta in upstate new york for members of the US national team in the mid 80s. Murray bicycles had the sponsorship deal, so the frames were painted with the Murray logo, but these bikes were (and still are) some of the finest examples of hand-built racing frames in the US. Rebranding is in fact a common practice, and still goes on today. These days, a new Serotta frame of the same quality costs between $3000 and $5000 US dollars, depending on tubing type, size, custom geometry, etc. That's JUST the frame and fork, no components whatsoever.
Other small framebuilders such as Tom Kellogg (spectrum cycles) made bikes for the US national team in that period and painted them with Murray. However, it's important to note that Alexi Grewal won the 1984 olympic road race on a Pinarello - an italian hand-made frame.
Hope this helps, and don't hesitate to contact me if you want more info.
http://thezenofcycling.blogspot.com
