The 1266 is really an upgraded (revised) version of the 1265.
The controls are the same except for the sensitivity control's pull and push modes were switched.
The battery compartment was changed on the 1266, as was the battery box cover. The 1265 had a little push button latch that was pretty easy to break. Also the battery packs in the 1265 hooked to two 9volt wired connectors, on the 1266 the packs drop in and make contact with springed tabs in the bottom of the box (no wires). Also the battery box cover is held in place with a screw knob.
The 1265 came stock with a solid coil. (boxed design)
The 1266 came with the newer spider coil.
The first change to the NEW 1266's electronics took adding an additional circuit board to the detector, a couple of years later technologically advanced enough to be able to compress the the 1266's electronic's back to a single board. Many people believe that the change to a single board caused some MINOR loss in depth to the 1266.
And like what has already been said they can use the same coils, not mentioned above there is also a 3-3/4" coil for them (also the coils will fit the 1270 as well)
Mark