The first F-111s, both As and Bs, were completed before the McDonnell-furnished escape module was qualified. The escape modules for these airplanes were therefore equipped with ejection seats. These were Douglas Escapac seats, probably Model IC. Note that they have two large pull rings for the face curtain rather than just a single one. My guess is that this modification was made to the seat at least in part because Grumman pilots were using a Navy HGU-20/P "clamshell" helmet.
Although it appeared to be a pressure-suit helmet, it was not (the F-111 couldn't get that high anyway). It did away with the oxygen mask and theoretically would be unlikely to come off in a high-speed ejection.
The ejection seat was much wider from front to back than the seat back in the capsule. Simply mounting the seat to the rear bulkhead of the capsule would have resulted in the control stick being in the pilot's solar plexus and him having to move the throttle with his elbow. In order that the he be properly positioned with respect to the controls, the seat's rails were located aft of the rear frame of the side hatch.
A panel, identified as the "overhead structure", therefore had to be added to the top of the fuselage to allow the ejection seat a clear pathway out. The edge of this panel is apparent in the picture above and in the following picture of an escape module which is to be completed with ejection seats.
The emergency canopy-removal sequence was complicated.
The canopy removal and ejection seat operation were qualified in a sled test conducted by McDonnell. (See https://www.youtube.com/watch?v=9gyIhOGyGA8)
The following still from the sled-test cockpit camera shows the canopy as it is being jettisoned prior to the seats going out.