Models of Yesteryear
All models were issued a 'Y' number. As the die cast moulds wore out so another new model was issued under the same number to take it's place, thus giving rise to, in some cases, four or five different models under the same number.
Models were coded as Y1-1, Y1-2, Y1-3 and so on for each number in the range. Because of this models were bought into the range on different dates and not as a full series in one go.
To go one step further, under each model is a variation number and so we get for instance Y1-1 -1, Y1-1-2 etc etc depending on how many variants are known on a particular model.