I found another stash of these:  Collins "Loaf of Bread" style mechanical filter, type 526-8445-010, USB, 500 KHz C.F., 3.8 KHz BW (shifted up from CF), input output load 2K ohms & 20 pF.  I also have the matching delay equalizer 524-5001-040 if you want it (you have to request it).  These are new old stock, roughly 1/2 x 1/2 x 2 5/8 inches, 2 threaded studs on bottom & 4 wire pins.  I have the matching LSB filters as well, so you can buy a pair LSB/USB!