For some reason, I couldn’t get a Numark cartridge to fit on the SM-505. It fit in well, but not fully set into the tone arm and there was no locking mechanism.
Do you mean something like Numark CC-1 or this one ?
Both are better in recycle bin than on a turntable, those are very bad quality cartridges, don’t waste your time with it even for djing.
Buy yourself a pair of Grado DJ-100 to mount on any lighweight headshell and use them for DJing or for home listening. When you are ready fo upgrade just buy a better stylus for this Grado.
If your Micro Seiki has some problem with headshell lock just send it for repair to anyone who can fix Micro Seiki. Your Micro Seiki tonearm designed for use conventional headshell, they are universal, nothing special.