Turn over the bicycle frame and look at the underside. You should be able to spot the serial number in this position. It is often on the bracket tube that is welded to the frame tube. Depending on the year, however, it might be located on the brace behind the bottom bracket.
Inspect the number to determine if it has five digits and begins with a 1. Mongoose frames manufactured in 1975 to early 1976 had five-digit numbers hand-stamped on them, starting with 10000. If the serial number of your Mongoose has a number such as 11994, with no letters in or near the number, it is likely a frame from 1975 or early 1976.
Scan the area to see if the letter C is etched above the five numbers. The letter C stands for Chatsworth, the city in California where the Mongoose was made during the early years of production. If there's a C above the serial number on your Mongoose, the bicycle was probably made in early 1976.
Look for a letter and number in addition to the C above the five-digit serial numbers. In May 1976, BMX Products began including a letter and a number with the C. The additional letter indicated the month the frame was produced. The letter A stood for January, the letter B indicated February, and so on. After the letter designating the month of production, a number appears that stands for the year of production. The number 7 indicates 1977, the number 8 stands for 1978, and so on. A serial number from that time period might show CB8 above the five-digit serial number, which is sequential and indicates the number of frames produced in a particular series.
Check for the letter X instead of the C with the serial number, or look for the letter T in front of, above or below the CB8 sequence. If the serial number on the frame has an X instead of a C, then it is likely a Super X model. If the frame has a T in addition to the other letters and numbers, it is probably a Team Mongoose, a Team Minigoose or a Supergoose.
Check for a three-digit designation that doesn't begin with the letter C. In April 1981, the company decided to drop the C and use only one letter, in the A to L sequence, to indicate the month of production, followed by the number indicating the year of production, followed by the sequential serial number. The only exception to this are the frames that were made in the company's Merida factory in Taiwan. These frames begin with an M, followed by the month letter, the year number, and then the sequential serial number.