CS-Bookends.
In the olden days, to keep books upright on a shelf and to keep them from falling off, one would use matching weights at each end of the shelf. These could be small statues, wood carvings, rocks, etc. Let us take two statues of horse heads as an example. One on the left side of the bookshelf would be looking left, and the statue on the right would be looking right or reversed from each other. Or, they would both be facing forward as a matched set. The same is the case with CS-Bookends -- they are reversed M 12xxxx21 M, repeated M 12xxxx12 M, or matched M 22xxxx22 M.

When two or three digits are reverse order from each other at the beginning and end of a serial number, M 47xxxx74 M, it is called a CS-Radar Bookend. When those digits are in the same order as each other at the beginning and end of the serial number, M 825xx825 M, it is called a CS-Repeater Bookend. When the digits at the beginning and the end of the serial number are identical, M 66xxxx66 M, then it is called a CS-Matched Bookend.

CS-Single Bookend
The digits in positions 1 and 8 of the FSN are identical.

CS-Dual Matched Bookend.
CS-Dual Matched Bookends have CS-Pairs at both ends of the FSN.

CS-Dual Radar Bookend.
CS-Radar Bookends contain two numbers that are mirrored at each end.

CS-Dual Repeater Bookend.
CS-Repeater Bookends contain two numbers that are repeated at each end.

CS-Tri Bookends.
CS-Tri Bookends contain three numbers that are matched, repeated, or resemble a radar at each end.

CS-Tri Matched Bookend.
A CS-Tri Matched Bookend M 333xx333 M has six numbers that match and are split into two sets of three, and the first set starts at position 1, and the second set starts at position 6.

CS-Tri Repeated Bookend.
A CS-Tri Repeated Bookend M 123xx123 M has three numbers that repeat. While two of the numbers can match, all three cannot. The first set starts at position 1, and the second set starts at position 6.

CS-Tri Radar Bookend.
A CS-Tri Radar Bookend M 123xx321 M has three numbers that are mirrored at each end of the FSN. While two of the numbers can match, all three must be the same.

Note: This page contains excerpts and basic information from the book "The Green Guide to Fancy Serial Numbers". Please consider adding a copy to your library, thank you.