Why is the story of how 3885 got established on 3873???
I can tell you about 3873........ This primarily came about because there is a net that operates on 3880 from time to time (I think it's the 1st Wednesday Collins Collector Net, which I have checked into a number of times), and other folks who regularly use 3880 at night.
So, what happened is that a bunch of operators who have very good antennas and reasonable power were regularly using 3875 at night, and particularly on those nights when the 1st Wednesday net was also operating, there was (as you can imagine) a lot of interference between the two frequencies. So, after quite a bit of back-and-forth, the guys on 3875 moved down a couple of kHzs to give the CCA net (and 3880 in general) more room.
It was not feasible to move down further, due to an established group of SSB operators who use 3868.
I find it quite reasonable to leave at least 7kHz spacing between AM stations. 5kHz is too close. 10kHz is even better, but is sometimes not practical. For instance, on Sunday mornings, if I am in the "window", and if 3873 is busy, I will often operate on 3887 to give more room for the DX-60 net. This does not get too close to SSB operators who are often on 3895 (using LSB).
