I found recently when adding the silent fill o rings that none of my mags had o-rings to take off in the first place and the actual valve been too deep for me to get anything into get the o-rings on... to solve this all i did was rest the o-ring on top of the valve and use the gas can to push it down and into place, so far all my TM mags are now silent fill and had no issues with over spill
PS. Those o-rings are £3 from fire-support for a pack of 8. I just ordered some today for my own mags.