matchbox-superfast trucks

[McColm]

Hello,
Unlike the pic below, my Matchbox Foden six wheeler concrete/cement lorry/truck is a bit tattered and chipped. The yellow cab with its green plastic has seen better days, the six wheels still work although one is bent and the red paintwork looks okay from a distance. Yes still got the yellow plastic tumbler, sorry no box.
This got me thinking as I turned it over in my hands the number 21 stamped underneath was that taken from the Foden S21 six wheeler series?
Did anyone have numbers 20 or 19?