What year is 15 hp mariner motor serial number 684CSO47666, I am in Australia..pa

Nothing in the Mercury Mariner Outboard engine model numbers or serial numbers provides year information directly

Hey Peter, try these guys.
https://www.marineengine.com/

Locate the serial number plate by looking on your motor's transom bracket; the plate will have "Mercury Marine" written on the top. The serial number is the top number on the plate. The number directly beneath this is the year it was manufactured.
The year will be determined by the number of cylinders,,,
A 4 cyl. is a 1986 .
75 - 4 Cyl. 1986 ELPTO, ELO A197882 - B249479 = U.S. Serial Numbers A70708 = Canadian serial numbers

the serial number begins 2487558 and continues past 2491265 and as the beginning serial number denotes the year , your engine is 1969 model
it shows as a 2 stroke with auto oil injection
check the heads
if the plugs are in the flat of the head and there are no rocker covers it is 2 stroke
talk with a mercury /mariner service center

Hello,

Mariner 4hp are identical to Yamaha engines.
You should find a sticker on the port bracket with serial and model number.
If that sticker has disappeard, an aluminum piece is stamped in the side of the cilinder. The numbervstamped in that round pieceis the serial number. I can convert that serial number in a year/model number...

According to i-Boats, "Nothing in the Mercury Mariner Outboard engine model numbers or serial numbers provides year information directly.
The first serial number sold in a year becomes the beginning serial number. Serial numbers between that year's beginning serial number and the next fall into the same year. Search for your engine horsepower on the Mercury Mariner serial number year chart located here to find the beginning serial number for the range that includes your engine." Your other option is to take the serial number to an authorized Merc marine eager and they can look it up for you.

call mercury outboard they can tell you the answer to your question

yes if yuo go to iboats.com the forum will allow you to search by serial number and give info on your outboard.