Gravely placed serial numbers all over the tractors at different times.  Some were riveted on, some glued on, and some were just foil stickers.  Here are the most common locations.  Once you find your serial number, look here to find the year.

L series tag above gas tank.

These tags were on all L series tractors. In the begining, they were brass, then sometime in 1948, they changed to aluminum. A lot of people will refer to an old tractor as a "brass tag".

C series located on the left side of the advanced casing.

When the C series came out, the serial number was relocated and rivited on.

Commercial series is the same location as the C series.

Just like the C

400 series riders located on the left side of the advanced casing.

Just like the C and the Commercial tractors.

500 Series is a foil sticker on the dash top

5000 series is a metal tag located under the hood above the dash.

Early 800 series located on the engine mounting adapter plate.

I'm not sure if this applied to all early 800 series or just 816 tractors.

Late 800 series, left side of lower dash.

This is the easiest to find and the easiest to lose. Some are glued metal tags and some are silver stickers.

8000 series, same as the late 800.

Same as the late 800.

G Series Has a seperate SN amd Model number metal tag.

Pro Master series has a SN metal tag in front of the seat and a metal model tag lower on the frame.