This should be fun (RCA TM-10 15GP22 monitor)

[old_tv_nut]

"Its ID plate says MI-40206-A, but the ETF website says it's a model TM-10, so that's what I'll call it."

RCA professional gear often had a type number (TM-10 in this case) and always an MI number that appeared in the catalog book/price sheet for ordering. Even the crossover networks used for the public address/music speakers at the 1964-65 New York World's Fair were specified by MI-number, which allowed me to track them down to determine the circuit used.