Sounds similar to the API 312/512 series preamps. They are totally underrated. I've used it on guitars, drums, flute, sax, and all kinds of things and it sounds great for all of the things you would use an API preamp for. The best part about these is that they can be found way cheaper than the API. This is an older one that was just called the "MP1". The input transformer looks to be of the Cinemag style (although OSA would remove the labeling) which would make it more like the MP-1A. The output transformer is the standard model that was custom-wound for OSA. This one has a 1/4" line-level input on the front. I have several other OSA preamps so that is the only reason I am parting with this one.
I recently recapped it and added bypass caps (in theory they allow high frequencies to pass easier resulting in more "air"). I replaced the gain knob with a bigger one too (easier to position). This preamp does require a 500-series compatible chassis.
***SOLD***
$325 shipped in the US. I will eat the paypal fees too. yum.
that is so cheap...