The make up of the Pultec HLF-3C is really one of the simplest possible, as with a lot of Pultec gear they have made it as easy as possible to use. The HLF-3C simply has two filtering bands, each consisting of only a single knob. There are both high and low cut off knobs to set your frequency, which you've got about ten frequencies to choose between on each of them. So essentially you've got a high and low pass filter here with the most basic make up possible. I don't know anything about the manual and would think that it would be quite hard to find even if you did want to look at one - I can't see why anyone would need to though!
The sound quality of the Pultec HLF-3C rivals the best high/low pass filter combinations that I've heard. In fact it might just be the best. It has an extremely clean sound and will do it's job well, but as with all Pultec gear there is a certain quality here that isn't found in any other units. It is something that is sort of hard to describe in words and really needs to be heard for yourself. If you've used other Pultec gear like the classic EQP-1A, then you definitely understand what I'm talking about. The HLF-3C is versatile in the sense that it can be used for filtering any instrument or voice really, but obviously doesn't have all the control in the world...
I really don't have a bad thing to say about the Pultec HLF-3C. While there aren't many parameters at all to play with, when doing high and low pass filtering, this is basically all that you really need anyway. I can't say what the price range would be for a HLF-3C, but it will of course be very pricey since these are especially hard to find and are coming from such a renown manufacturer. Having said all of this, if you can get your hands on one for even a day, I'd definitely recommend doing so!