Dunlop Special Lightweight rims - Tubs or Clincher?

The answer I got back is below. To put in context, the reply is from an Pom, now resident in Australia

[INDENT]il padrone wrote:In the 70s, before the hook-bead rims came along a high pressure clincher was 80-90 psi.
In the 50s......? Maybe 70 psi, I don't know.
[/INDENT]Have to agree with Il Padrone (getting to be a habit - worrying.... ) Back home we used to run our Michelin on 'plain' Weinmann rims at a max of 90 psi on the tandem. In summer you were careful not to leave the bike in the sun for any period at this pressure or you risked a blow-off . Also heavy braking on long downhills on a tandem could heat the wheel up to the extent you blew off the front tyre (reason for rear drums....).
I wasn't around in the 50's but the old guys in our group who were used to run 80-90psi in the tyres on their Dunlops. However they did say that clincher tyre quality had improved so much between 1955 and 1975 and that they wouldn't have risked that pressure using the tyres available then. I didn't bother at the time finding out what that pressure was.
Also, I used to commute to work in London on an old Raleigh 'racer' which had these rims and in my ignorance I ran them at 80psi with Michelin World Tours and had no problems (of that sort) at all.
I wouldn't put a folding tyre on an unhooked rim mind you, that's asking for trouble
Sheldon Brown also advised

Traditional 630 mm (27 inch) rims were straight side design, but in the late '70s they evolved to a "hook edge" design which would permit the use of higher pressures. These days, many 630 mm (27 inch) tires are marked "For hook edge rims only" (some companies use the term "crochet type" instead of "hook edge"...this is the result of poor translation.)
The fact is that modern 630 mm (27 inch) tires will work on older straight-side rims, but they won't handle as much pressure as they are capable of with hook edge rims.
Generally, the "rule of thumb" for traditional 630 mm (27 inch) tires is that they should be inflated to 70-75 psi. This shouldn't be a problem with any tire, despite whatever disclaimer the manufacturer puts on the sidewall. However, if you're restricted to this pressure range, you probably shouldn't be running tires narrower than 1 1/8, or preferably 1 1/4, unless you're a very lightweight rider.

on that page there are also the measured widths and there appears to be substantial variariations the the actual widths (needed for calculating the tyre pressures based on tyre drop).
So I will be buying some rigid (ie non-folding) 27x1 1/4" tyres.