I'll bet they're a 921 also. Some of mine were darn hard to pull out and put in - wiggle and pull. The cover slides out then pulls down on mine to align the retaining ears with the slots. The link below is what I used for replacement and they have done wonders; heat is almost non existent. My converter used to kick in when I turned on the first light switch. Now I can turn them all on with the exception of the bathroom (1fixture) and you can't hear the inverter. Good luck.
http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1