I suspect the original paint on my Humber Pig gearbox was yellow zinc chromate. Although with time & grime it is not easy to be certain of the original shade of yellow!
The only thing that fits the bill in terms of zinc chromate, colour & use that I can find in VAOS & COSA is specified as Lemon BSC 355.
I see Halfords do aerosols of zinc primer in various colours, one of which is a bright yellow judging by the plastic cap. Has anyone used this? Does it look like the desired colour? All other outlets for zinc primers are red oxide & grey, even some that are water based which doesn't sound too durable.
I checked which Halford stores had it in stock, but the stock checker is none too clever as you can't check which particular colours are in stock.
http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_173576_langId_-1_categoryId_255239