This is a typical example of the final variation of the 991 from 1954 to 1956. There is no conclusive proof that the model stopped in 1956, but no known examples exist after this year and they are not mentioned in a 1958 catalogue. The main difference with this variation is that the model number is not stamped into the tank but is on a label on the collar. This was to enable the tank to be used for both the 991 and the 992. The top is also simpler than before with no internal cone and therefore no top nut. This 991 is from 1956 and is stamped AV z underneath. The label is stamped 101656. This would indicate that it was made (assembled) in week 16 of 1956. I have included a before shot to show the 'lovely' gold paint that a previous owner thought looked good.