Number Three...Frame Number
If you own an MK1 Chopper, an early MK2 Chopper or a Chopper Sprint, you’re stuck with the rear hub date as identification, All sprints were produced 1972 1973.
These early models had the frame number consisting of seven numbers... although some late early models have only six!
I have found no way of dating these models... although they usually fall within a production run of only a couple of years depending on model.... And most U.K. issue bikes would have had a Sturmey 3 speed rear hub to date.
All MK1 and most MK2 up to late 1972 have their frame numbers stamped on the left rear dropout.
This is the bit of flat steel that the rear wheel bolts into, on the other side to the chain. To read it fully you often have to remove the rear wheel nut and washer, and of course clean off the dirt or rust.

.....
This is the same bike as the hub on the previous page

Late 1972 and on bikes had their frame numbers moved up to the seat tube, under where the seat clamps onto the frame, running up the tube from bottom to top.
The later frame numbers consist of two letters, followed by five six or seven numbers. This is where it gets easy...
The FIRST letter (usually "N") indicates the factory the frame was made at... obviously most U.K. bikes originated from Nottingham... hence most of our bikes start with "N"
The SECOND letter indicates the month the frame was built... this letter should relate to the rear hub date within a couple of months either way, depending on stock levels at the factory.
Here’s the list;
January A, February B, March D, April E, May G, June H, July K, August L, September M, October N, November P, December S.
The THIRD in the sequence is a number, this indicates the year of production, 4 is 1974, 5 is 1975 etc And 1981 is 1, 1982 is 2. (It starts with a '4' because this type of numbering only started in 1974)
The rest of the numbers in the frame number are the bikes serial number.
Therefore, a bike with frame number ND46057 Would have been produced at Nottingham (N) in March (D) in 1974 (4) and be frame number 6,057 produced that month.

............................