I've found some information on this trumpet that I thought was interresting. These are others comments, not my own. I haven't verified the validity of any either.

********
The Pinto was introduced during the Arab Valve Oil Embargo of the early
70s as an economy model. The plastic valve jacket is lightweight and does
not corrode, meaning that the horn used far less valve oil than the
heavyweight "valve oil guzzlers" fashionable at the time.

The Pinto was recalled in the late 70s, when engineers concluded that
inserting a cup mute quickly by plunging the horn onto a  mute sitting on
the floor caused the third valve slide to spontaneously explode.
********
> The eBay auction closed at 31.05US a few days ago.

That's probably about 31.03 more than the horn is worth.  The Pinto was
an inexplicable experiment in cheapness by Olds, completely out of
character with their other horns.  Plastic valve casings?  Give me a
break.

My guess is that Olds was having difficulty getting the manufacturing
cost of the Ambassador down low enough to compete with Conn, Selmer,
and Holton, and so they made a desperation attempt at developing a horn
they could mass produce cheaply to keep them in business.  It's a shame
they didn't come up with a better solution.
********
I had the opportunity to actually play one of these horns about 3 years ago. 
A friend of mine was repairing one.   The "Pinto" was an idea gone sour.   The
Pinto was designed to be an entry level trumpet that was virtually
indestructable.   The brown plastic molded valve casing jacket that you
describe was actually the valve casing!   The slide tubings were actually easy
to remove from the plastic casing.   the same goes for the bell.   The valves
were well protected by the thick plastic casing.   If a tubing was dented,  it
was a snap to simply replace it!   The concept was ingenius.  An inexpensive
trumpet,  easily repairable( just replace the dented tubes), no worry of
crushing the valve casing.   What could be better?   The problem?  It sounded
awful, and the tubing was known to fall out of the casing.   There weren't any
solder joints holding the horn together.   The horn makes a great collector's
find,  BUT if you're planning on playing it,  you'd be better off with the
Ambassador.
Sincerely,
Wayne 
PS -IT'S AWFUL LOOKING TOO!
********
From Alan Rouse's article on working at olds.

One day they gave us a new design piston to make that had no 1,2 or 3. One piston would fit all three valves. It was interchangeable and was not stamped with a number. This would be the Pinto trumpet .Not only were the valves interchangeable but also the braces would unscrew and the bell and leadpipe would come off without unsoldering. In order to make the valves that way they had to change the position of the holes in the piston. This made a problem for us. The top and middle holes through the piston were too close together but they had to be that way to make the valves interchangeable. When balling out (expanding the tubing on the balling machine) there was a big hump in the bottom of the top crook. It looked awful but there was no way to get around it. You had to push so hard the piston would bend of the crook would split. We had to throw away up to half of the pistons we made for this horn. The one that was bent had to be junked however we could save some of the tubing splits by soldering them. We hated to make these pistons. They were the hardest to make. Also when we repaired them it was always on no bonus time .Any repair time was straight time .It was a big problem that they could never get around. So here is what they did.

I picked up an Olds brochure at the music store about the Pinto and was surprised to see they had highlighted the large bump in the top crock. Why would they do that I wondered when they tried everything they could to get rid of it. They gave it a name. They called it “ turbotron “. The brochure said the horn played so well because the “turbotron” compressed the airflow then forced it out at increase speed and there fore improved the sound. It was a brilliant concept to take a problem and turn it into a plus. The Pintos did not do very well and we did not make many of them.