The screw used to tighten the tremolo arm on the JP 157 uses the same allen key as the volume knobs. I lost my tremolo screw and tried to use the screw used by the volume knobs but it just doesn't go in. re the screws slightly different in any way?
Give EBMM customer service a call (or email) - they'll send another your way.
You should contact Sterling by Musicman to find out what the specs are on the tremolo screw. Just because they both use the same hex key doesn't mean they are the same thread pitch or diameter.