Many thanks for all the valuable contributions on this topic. I always like to stand corrected on such things. This one had me beat for a few reasons.
Interesting that HGP used the Walter Barr wheels with the central red caps. I'd only ever seen them on Southern Cross and old Rover utility mowers, but the ads clearly show them on HGP's too.
It's interesting too that I've sometimes seen old Rover mowers with the same wheels that HGP commonly used in the mid-60s, with the larger plastic caps. H.G. Palmer always had their name on their caps, other brands like Rover and Supa-Swift had plain caps. The Southern Cross KXA had the same wheels too, but the colours were reversed on them: red hubs, snow white caps!!
In addition, it seems that Kirby-Lauson engines became the more favoured engine on HGP's in the later years. At least I tend to see more old HGP's with Kirby-Lauson engines than ones with Briggs engines.
