i tried looking up the serial number you posted in hypass and it comes up as not a good number, it designates the prefix as D003
If you look on the right hand side of the hood area on the frame the serial number should be stamped into the frame there, just verify that it is the same as what is printed on the dataplate.

Based on the information i can get by looking up the service information by model number this is what i determined (and ironically it shows the serial number breakdown as D3 prefix in it's example) go figure.

But anyhow if the information is accurate
D3S-3268A
D3 = Design Series H40H thru H60H
S = Mfg'd in Australia Plant
3268 = this truck is the 3268th truck built in this series
A = designates the year of mfg

now as for the year the manual indicates it as a 1957 year of mfg which we can probably rule out, and the ending letter Z designates the last year in this run at 1979. So effectively the letters start over at A and the next year is 1980. So A could very well mean this IS a 1980 model.

verifying the serial number by whats stamped on the frame would help alot, also can you describe what engine is in this unit? As you said, the distributor has a oiler port? with a flip cap on it? This makes me wonder because i havnt seen one of those in 30yrs so i do have my questions about the year model.
I doubt it is a 57 model but based on that oiler on the distributor and the engine which would most likely be a Ford 4-203 engine? I have seen some continental engines in those older lifts too. Verifying this could prove most helpful in determining the right year as well.