If of interest, Cobham lists the following for the Mk3: Wire 3.25mm, Coils 29, Od 21.5mm, id 15mm, length 210mm.

Also another snippet for Titan: The Titan No. 7 is listed as fitting the HW50, 55, 57. I tried one in my HW55T restoration project and the original spring guide ... a typical early HW steel cross end jobby ... would not fit into the spring. No iffs or buts or pushing ... the spring had too narrow an id and it would have needed the original guide reducing to make it fit. I did not do this but sourced a proper size spring.

Cheers, Phil