I have an interview on tape with Frank Lester W4AMJ (then W2AMJ) who was Mr.
Hammarlund from 1955 to 1970. When he was hired by Lloyd Hammarlund (son
of the founder) he found the Pro-310 did not come close to specs. He and
the Engineers spend quite a bit on bring it "close" to apecs.

The reason you don't see many is that the turret costs $85 per unit. That
is what there is the box within a box. Hopefully the band switch cable has
only fallen off. Otherwise its a nice box outside and a real dud inside.
Lloyd put a stop to production after only a few hundred units!

Frank found that none of the Engineers were hams. Since Lloyd cut off the
funds Frank went to work to get some of the Engineers to be hams. He went
to work on what saved Hammarlund (outside of several commercial ventures)
and that was the HC-10/SPC-10 SSB/CW converter. Frank wanted to use crystal
or mechanical filters but Lloyd asked for alternatives (they were
cheap)which is the principle reason for the 60 kcs IF's found in the HQ-170,
HQ-180, and the HC-10. Lloyd kept the old SP-600 series alive by selling
the SPC-10 at several times the HC-10 price to the thousands of users.

But never fear, the idea behind the Pro-310 stayed alive in the guise of the
HQ-215.

Hope the Pro-310 turns out to be a great piece of History. Frank could
never say why the H emblem was missing from most of the units.

73 Dave K4JRB