Not sure about "175"
The Elkin boxes never used numbers, Just Elkin 2 tube, 4 tube ect.
So if it is mobile they made a 2, 4, and 8 tube.
Pretty good old amps. They have not made any amps in 10 to 15 years. And were made here in NC. So if it has 175 on it, is that stamped on the unit? If so that is the production number and would make it over 20 years old.