The 257 is used with the 258 and was designed exclusively for use with Thomas organs.
Below is a description by Harvey Olsen from the “HamTech Mailing List”:
“The 257/258 dual cabinet system was actually for selected, high-end, 60s vintage Thomas Organs.
The 257 was used for the flute and tibia voices. It’s essentially a 247, but with a nonstandard 9-pin input for Thomas. If you run into a 257, the 6550 power amp is easily rewired to 6-pin, 147 specs.”
