Pablo, as per our recent email conversation, I also cannot figure out why the 103R won't fit in your arm, because I used it in my OL arm, and I also use a Shelter 501. I'd like to know which "dimension" of the 103 is "too big" to fit in the Ekos. It fits in virtually every other headshell that I know of, and has standard screw spacing. I have never heard of any 103 being "too big" for any headshell.
However, if you are "forced" to use a Shelter 501, this is not a bad thing, because is is better than the 103R anyway.
However, if you are "forced" to use a Shelter 501, this is not a bad thing, because is is better than the 103R anyway.