You've answered your own query with the Autodata extract Alan. It would bear checking out as a minimum. To paraphrase Autodata, get the engine up to operating temperature, remove the vacuum advance from the distributor and plug it, turn the slow running screw all the way in (count the turns in just in case) then turn it 4 complete turns out and start the engine. This should give an idle speed between 10-1200rpm.
If so, unplug the vac line and reconnect it to the dizzy then turn the slow running screw back in to achieve the 900rpm idle speed.
If not, bend the tab to bring the idle speed to within 10-1200rpm (i'd aim for the mid-point of 1100rpm) and then follow the step above.
You're probably wondering why i'm viewing this as important. Many moons ago i ran a Mk1 Cavalier 2.0 which had a Varajet-II carb. This had a similar system of setting the throttle butterfly to a specific point and then adjusting the idle speed by means of the idle air bypass/slow-running screw.
On that carb, the slow-running screw had to be turned all the way in then with the distributor vacuum hose disconnected from the carb, a water manometer connected onto the vacuum advance stub on the carb and the idle speed adjusted to 8" of water vacuum in the water manometer. This was the mid-point of the acceptable range, i don't recall the entire range now but it made a huge difference to how the car ran overall including performance, economy and lack of flat spots.
Looking at the information on the Pierburg carb, not just above but also what limited information i found online, i would conclude there is a similar arrangement on yours - obviously without the need for a water manometer to set it correctly and with a tab to bend instead of a screw to adjust the idle speed.
It may be after checking it you conclude the idle speed is correct with the slow-running/air idle bypass screw turned just 1/2 turn out but until then i would say it is suspect, given all the previous misguided fiddling that seems to have happened over the years. Don't forget the tab is designed to be bent a few times at least so i wouldn't worry about breaking it.
Up to you of course but for peace of mind, i'd check it out.
