The numbers on the bottom of the cases are commonly called the “belly” numbers or “line bore” numbers, and they should be under the flywheels. ¹⁾
If your bike has “born together” cases, the numbers on each case half should match.
These numbers have nothing to do with your gasket set or anything else.

Other relevant numbers on the motor is the VIN, also called engine number or serial number, and can be found on a pad in the “V” between the cylinders.
Motor numbers are on the left side of the motor from 1957-1969 and on the right side of the motor from 1970-1985.

From 1957-1969, motorcycles were titled to the engine only. Starting in 1970, motorcycles were titled to the frame instead of the engine.
But if your frame and motor were born together the numbers will match (and possibly save you title hassles).

There are also casting numbers in various places on the engine and frame, but they are not motor numbers.
These do not indicate the year of your bike, only the date the cast was made.
So, for instance, on a 70s bike, there are still quite a few -52 (the first year of the K model) numbers.
(leading the uniformed to advertise all sorts of later model parts as K parts 0

Decoding the motor number / VIN will tell you the year of the motor and hence the gasket set you should buy.
Often, riders add later parts, especially items like transmission doors and oil pumps because the later castings have been improved.
Just because you have a later casting number on a part of the motor does not mean you have a later motor though.

Both case halves should carry a set of numbers that indicate their actual machining date (when the case halves were machined as a pair).
Belly numbers have no relationship to the Serial number (or VIN number respectively) located on the side of the engine.
Belly numbers are stamped the day the case halves are machined (line bored) and paired up.
The motor serial number or VIN respectively happens much later after engine assembly.
These two have absolutely no correlation to each other except they can be a dead give away on bogus numbers when they are out of the realm of possibility with each other.

From 1957 to at least 1969, belly numbers were coded as: Model (1 digit)-Year (2 digits)-Sequential No (4 or 5 digits)

Sometime in the 70's, belly numbers changed from simple sequential serial numbers to include the Julian Day Code and sequential production number for the day.
Model Designation (1-2 digits)-Year (2 digits)-Julian Day of the Year (3 digits)-Sequential Number (3 digits)

• The model designation for 1000cc Sportsters was (7).
  • Example: Using belly numbers 776 015 024, the code translates to;
    • 7 = Sportster, 76 = 1976, 015 = January 15, 024 = 24th production of the day.
    • Both case halves would have the same coding indicating that particular case set was machined (line bored) on January 15, 1976.
• The Julian Day starts over on every January 1st.
  • Example January 1st = Julian Day #1. December 31 = Julian Day #365 of a non leap year, December 31 = 366 of a leap year.
    This is verified thru Service Bulletin numbers 744 and 760.
• For convenience, Click Here to see the Julian Day Calendar page in the Misc Resources section of the Sportsterpedia.
  You can open a pic of a full per-year calendar thru 1985 there and match your belly numbers to find the day your cases were machined.