I see what you mean Buzz,I wonder if it was just the early DH Dragons with the external RPM gauges.

I cannot find my own Dragon Rapide pics at the moment but I have borrowed a couple of pics from net  – maquettes

https://www.net-maquettes.com/pictures/de-havilland-dragon/ which show DH Dragon ‘IOLAR’

You can see the ‘teardrop’ cutout in the top rear of the inner engine cowl

Close up of RPM and Oil Pressure gauges

rgds baz