A clincher rim uses a tube and tire and is attached/locked in to the rim via the outward pressure applied by inflating the tire. A wire or kevlar cord, the bead, sits in a groove in the rim. Early clincher rims had no lip.

Tubulars/sew-ups are glued on, no bead, no groove, no seperate tube. They can be used at much higher PSI than most clinchers.

Now there are tubeless clinchers.

I'd be shocked if Weinmann didn't offer 27x1" rims.

Not sure what your asking in regards to your Weinmann rims being high pressure. Any decent quality clincher rim will hold up to any kind of pressure the tire being used will stand up to.

If I were you I'd run that Campagnolo/Weinmann wheelset you've got laying around.