Rare volume knob version of the BSM BM-Q (Brian May inspired boost).

If you're looking at this listing, you likely know what it is.

Here is some info from the BSM website:

Brian May from Queen used in his early days the British Range Master Germanium Treble Booster, a unit produced between 1965 and 1969. This model is rebuild as RM by BSM. In mid-phase from Queen, BrianMay switched to a silicon Treble Booster design for more bite, now reproduced by BSM with orginal silicon transistor and called BM-Q special. The same unit fitted with volume control called BM-Q.

Almost all British rock guitarists using single coil pickup guitars used a germanium Treble Booster from the late 60's to the mid 70's. By the end of the seventies, the Treble Booster was replaced by a new circuit from Japan, the so called Tube Screamer and other similar overdrive circuits. These were based on the old Treble Boosters and therefore had a very similar frequency response. The boosters on the other hand, sadly fell into oblivion despite their unique and inimitable sound. The BM-Q Treble Booster has been specifically designed for single coil pickups with a standard coil inductance, such as pickups in Fender, Rickenbacker, Burns or Gibson guitars. On these pickups, the BM-Q produces (with the volume control of the guitar turned up to the max) a fat, biting and penetrating lead sound, minus any shrill characteristics. By lowering the guitar's volume control, many shades of crunch can be easily dialed in. Another thing to consider is that when a guitar amp's volume is turned up (reaching saturation), the power amp normally begins to mute the highs, which results in a duller sound. At an amp's full tilt, the BM-Q Treble Booster allows a more transparent tone (covering the entire frequency range) to shine through. The presence control on the amp need only be minimally used (if at all). When vintage amps are used (such as the Fender Deluxe, Fender Bassman, Vox AC30, plexi Marshall 50 or 100 watt) it is amazing what effect the use of a good treble booster has. It is also of interest to note that Brian May used a couple of VOX AC30 combos with the BM-Q in front of the "Normal" channel by volume setting three quarter of full. If you don't have an AC-30 amp, you can simulate the behaviour of it's CLEAN channel with any other tube amp by plugging in your axe into the NORMAL channel + cutting down the TREBLE pot while open the MID pot all the way up. Adjust the BASS control to your own preferences and you are done.

The Treble Booster is inserted between guitar and amplifier, not into the FX loop. The magical tone is achieved by the interaction between guitar pickup, treble booster and amplifier. The unit is powered by a 9V battery with a current consumption of approx. 500 uA. The average output level is 10dBm, the maximum output voltage when the strings are struck really hard is 7V max.

The Company - BSM Workshop

BSM uses only New Old Stock germanium transistors manufactured 30 to 40 years ago in Germany (Valvo, Philips, Tungsram - models OC44 or OC76) exactly like the original Treble Boosters that are much sought after by tone connoisseurs. Other manufacturers use more commonly available non-original germanium transistors like the AC122, AC128, AC153, 2N508 and NTE102. The resulting treble booster circuit is a bland clone - minus the original sound. The NOS germanium transistors cost many times the price of the more commonly available AC122, AC128, etc. In addition, BSM carefully selects the OC44 (about 50% are rejected) and OC76 (about 30% are rejected) used in its units. After careful assembly and thorough testing, the circuit is soaked in a ball of resin and allowed to dry for 3 days. This ensures that the circuit will not be subject to outside temperature fluctuations which made the originals notoriously unreliable.
For the tranny freaks, here are some more info about the introduced transistor types in Germany/Europe: 1954: OC70/71 / 1955: OC72 / 1956: OC73, OC44, OC45, OC76 / 1958: OC75