Hi there Alan and Sapper,
very helpful ideas from both of you.

I used the relatively pristine underside as my guide and it is very much off white, further confirmed by my removal today of the label off the front for the first time since leaving the factory several decades ago (see pic below). It was like archeology lol.
I was fortunate to come across a can of Plasticote brand paint with a near dead on match compared to other beige like colours I directly compared it to. It'll look incredible once painted up and fitted to the mower!





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Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!