The only way to really accurately determine the year is via the chassis number, which is inaccessible unless you remove the cab. The number on the registration, if it is the original engine number, will tell as well. If the original engine is in there, that number will also tell. But engines get swapped. If the nose piece of the splash apron is spot welded to the fender, then that is a 30 fender. If the nose piece is bolted, so the fender can be removed without lifting the cab, its a 31. Maybe...after all these years things get mixed . Additionally, an early 31 would have any left over 30 parts on it. The one wheel you have on the drivers front is a 30 wheel. The others with the ribbed hub are 31. The 30 wheels cracked and were often replaced. Also, when using dual wheels, often using 31 wheels allowed for larger tires due to the extra space created by the recessed wheel design of the 31 style wheel.
Its a pretty decent truck to start with. Get it running and enjoy it. Don't tear it apart thinking you will restore it. Its not uncommon for a new owner to get excited about restoring something and tearing it apart, only to find a day's work taking a truck apart requires many, many years to reassemble. Often the project sits in pieces for 40 years after that until its sold as a basket case. If you want to restore it, do it in baby steps so you can enjoy it in between steps..