Retro-rebuses are problems where you have to decipher an
encoded chess position, with legality as a constraint.
This kind of problems was invented by Frolkin in the 70's. It is a
generalization of (and includes) the earlier
Here is the earliest example :
A. Frolkin and A. Kornilov
Rex Multiplex 6, Apr. 1983
17. Associate a different chess unit to each letter.
(b) Ag1 -> b1
Here C and E are the only non-adjacent
single-occurrence letters, hence they must be the 2 Kings. Now
B must be a wh.P because any other unit would give
illegal checks to one of the Kings.
The rest is decoded in the same way. (See
full solution if you can't work
it out yourself.)