Thank you everyone! Great info. Yes, the MK1 switch is the Volvo 1800 overdrive switch, and certainly the car builder solution and ebay stores offer very good substitutes.
I am in search of the specific switch as used on the MKll. This is different and quite unique. It is pictured in the vintage advert Rob posted/attached. As Stephane mentioned the switch housing is a 'T' shaped housing, and secured to the steering column with a hose clamp.
I have searched Ebay many times, but nothing yet.
If anyone has seen them on anything, or with a horn set, please let me know. Many thanks.