The story is probably very boring; we can't know the exacts of why but it is probable that they just ran out of one type that day or the other type was in easier reach. It wasn't like anybody cared at the time.
Scarcity between the two stocks, and the two stocks themselves, change between series. The lighter stock can get really tough in some of the middle series. Some series may have 3 stocks. Series 6 is the easy one to spot the differences, as it is a bright white and a cream and not a pedantic difference. I collect series 6 both ways as a result, the others are rather difficult to collect buying online as it's hard to ID them correctly from scans in many cases.
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