[R-390] Litton Model 1309 Microdial ?

[K3DX]

Greetings, All!

At my radio club meeting tonight, a friend handed me a turns-counting dial,
in the box. It looks an awful lot like the BFO dial on some R390(A)s.
But it has no reducer. Given that the dial shows 100 units per turn
(out of a total of 1,000) and that the R390A BFO knob only rotates through
180 degrees, how useful would this be? I would get 25 units on either side
of BFO center frequency. Is this what the "real" counters do on the R390A?
And why do some R390A's have the BFO counter dial, while others do not?

73, Dave - K3DX     Delaware's "DX" Station