Hi Rad,
First, welcome to the forum.
As to your question, I don't know how helpful this information will be but it is taken from Malcolm Bobbitt's book, Fiat & Abarth 500 & 600. He lists two models of Fiat 600 - the original 600 Saloon (Type 100) dating from 1955 to 1960 fitted with the 22bhp 633cc engine and the 600D (Type 100D) dating from 1960 to 1969 fitted with the 29bhp 767cc engine. The model change occurred halfway through 1960.
The Type 100 600 had a production run of 891,107 units (~160,000 per year) and as each chassis was sequentially stamped, I guess your car at chassis number 789 888 was fairly late in the run - likely a late 1959 or early 1960 model. I can't be more specific, sorry.
Of interest, the 600D Saloon was produced at a similar rate over nine and a half years yielding a total run of 1,561,000 units.
The 600 Multipla and 600D Multipla were produced in much lower numbers.
As to the owners manual, I think someone posted a couple of links to these a little while ago on this part of the forum.
Regards,
Chris