Short answer: bad flasher relay. I had another 1974 and swapped relays. Dash lights behave properly.

Longer answer: I looked at the circuit diagram for a 1974 and it looks as though the turning indicators aren't part of the bulb integrity system. Strange.

Also, admittedly from a very small survey, the flasher relay for the 1974 is unique to that model. It has 4 contacts. I had a look at a 1973 and also a couple of 240s; Their relays all have a 3 contacts although they use the same wiring harness connector. The 4th position, marked "C" in the 1974 relay, is left empty. I tried one of these 3 contact flasher relays. It worked for the outside lights but not for the dash lights. The reason is obvious when you look at the circuit diagram.

Regards
--
JohnH, Sydney, Australia