Modifying Replacement brushes on Braun Oral B toothbrushes?

Modifying Replacement brushes on Braun Oral B toothbrushes:
I have two older Braun Oral B toothbrushes (NC Accu) which still work just fine, but have had some challenges finding the 'right' replacement heads. I think it has to do with Braun and patents.

Neither of the last packages of replacement heads is a perfect fit on the unit (although they fit all Oral-B units except for Sonic according to the package). If you look at my unit, the upright portion has two symmetrical indentations. One of the Braun replacement heads has four projections, two on each side. I found that by CAREFULLY using a small chisel or file I could remove the extra small one on either side and the modified head fits fine.

The second package of replacement heads has only one indentation, which matches one side of my toothbrush. It does fit on the unit but I suspect it is less stable. I wonder:
a. Has anyone else had these fitting problem between older Oral B units and newer replacement heads?
b. Is there a better way to modify the replacement heads?

I like the units, and will buy another Braun toothbrush when thee go, but they still work fine. And I have spent the $20 on a set of newer replacement heads.