Thanks everyone for your contributions. Here's findings so far, though very preliminary ...

Examples seen / received from 15 $1's, 8 $5's and 4 $25's. Not a large sample, but a start.

The 3-digit numbers: appears that all numbers may have been used (OOO to 999), though specific examples range from 012 to 990. So ... it's possible that for each letter, there could have been 1000 serial numbers per denomination.

For the $1's (14,000 made). One theory is that the first 14 letters in the alphabet used (A thru O). But there are Q's also. Perhaps they used A thru Q and omitted letters I and O, since they look like numbers 1 and 0 ??? Speculative.

The specific letters seen for $1's are: D G H K L M N Q. Note that A-B-C not seen: Why? Perhaps these were made, but were always vault chips, only to be destroyed ultimately, since A-B-C used with the $5's and $25's ?? Also speculative.

For the $5's (4,000 made). Only the letters A B C and R were used. These 4 letter could account for the total number of chips made. Besides starting with A-B-C, note that the other letter (R) comes after the last letter (Q) seen on the $1's.

For the $25's (1,000 made). Only the letter A is seen, though only 4 examples. Maybe the only letter used for $25's?

Thanks again. If you see more examples, I'm always interested in hearing about them.