This item is a scarce Drake FS-4 frequency synthesizer used by many pieces of RL Drake equipment manufactured in the 70s and early 80s. It was used by me personally with my Drake SPR-4 for a number of years and I am downsizing my receivers due to financial need.
During the time of manufacture in the 70's, many of the Drake receivers were crystal controlled and the user was limited to about 500 khz within each crystal range. Because the receivers had a limit on the number of crystals that they could use within the receiver, the Drake receivers were not general coverage and the owner had to prioritize which bands they wanted to be able to tune into and sacrifice listening in on other bands.
However, Drake also offered the FS-4 frequency synthesizer for sale and this allowed for general coverage on many of it's receivers. As I recall, the original cost for this unit was around $250 or so making it quite an expensive accessory item at the time when the receiver sold for about $400 to $700. This explains why the production was limited and why these FS-4 units are not that common. For this reason, I have set the price to reflect this scarcity. If it does not meet with your desired price, I will be further reducing the FS-4 price periodically until sold.
The unit was specifically designed to interface with the "4" series of receivers like the SPR-4, the R4-C, and the T-4X. (R-4, R-4A, R-4B, R-4C. SPR-4, T-4, T-4X, T-4XB and T-4XC) For example, with the SPR-4 receiver, the operator can tune from 500 khz on up to 30 mhz. The frequency on the FS-4 in Mhz is added to the frequency on the PTO in Khz for the proper frequency reading. Thus, if the FS-4 reads 3.5 Mhz and the PTO reads 450 Khz, the frequency being received is 3.950 Mhz.
The FS-4 provides the proper injection signal using a cable that connects into one of the crystal sockets. The center of the coaxial cable plugs into the socket and the shielding fastens to the chassis of the receiver. When I used this cable on my SPR-4, there was a nearby screw to attach the shielding so this connection was a snap to accomplish. There is also a grommet attached to the cable to allow for a professional look after installation.
While not designed specifically for the "2" series receivers, the FS-4 will work with it. However, it is not full coverage as you have to add 7 mhz to the scale and this makes the FS-4 usable from 7 - 30 mhz on the "2" series units. You also may need to exchange the crystal inside the FS-4 with another one that is available online, but my experience has been that the calibration travel on the units generally allows for you to use the crystal as installed.
Here is information found at the Drake site regarding this unit: