Not yet found info on these units, but they appear similar to the MA-197B preselector.
The MA-197B and the MA-6197 both cover 1-30MHz in 6 ranges:
1-2, 2-4, 4-8, 8-16, 16-24, and 24-30MHz.
The externally very similar MA-197B manual is online here;
https://www.pa3esy.nl/ontvangers/racal/racal-algemeen/pdf/MA_197B.pdfThese things are of the highest quality, cast chasssis, etc.. a relay-rack dream. Signals more than 5% off-tune are attenuated >85dB, while desired signals are boosted 6dB. Pretty nice.They can also be daisy chained so one antenna can serve more than one preselector. The weight is only 45 lbs.
I have not opened the MA-6197 yet, but I have a suspicion. The MA-197B has 6 ranges, but it is more or less pass-through on the 1-2Mhz and 24-30MHz ranges, meaning those have RF protection but no pre-selection.
So, why would there be this mysterious model, the MA-6197, which only shows up in a few old 'for sale' ads with zero additional information?
Could this be an unusual model made for secret squirrel?
Could it be that all 6 ranges have pre-selection?
Waiting for cooler weather to check.