Good timing. I'm in the process of determining the maximum physical dimensions for a set of replacement capacitors, and I also had my really cheap autoranging DMM handy. There's a sticker inside of my amp's chassis that says it was tested on 6/17/75 and the reverb tank model, like yours, is O.C. Electronics P/N 29.0006 Type 51. The input resistance was about 175Ω and the output resistance was about 173Ω. At first I measured this with the tank connected to the circuit board, but then realized that parallel resistances could bring down the measured value. That being said, when I unhooked the tank, the measurements remained constant, which in retrospect shouldn't be surprising--why load the input or (especially) the output when you don't have to?
