The SL-5's (short) tonearm is built into the cover, so you may just have missed it (If it's really missing, it should be considered a parts machine). It's a linear tracking turntable, which means the entire arm moves linearly across the record as it plays, rather than on a pivot like conventional tonearms.
This section of the Wikipedia article on turntables has some more info. Technics didn't invent the LT turntable, but they certainly popularized it in the mass market. Before the
Technics SL-10 (their first relatively low cost LT turntable), linear tracking tonearms were very expensive.
Numark I don't know much about, but most of their stuff was generally on the budget end. I know at least one Audio Technica DJ turntable had a really bad tonearm design; it was straight but positioned in such a way that the tracking angle would always be terribly off. Getting the model numbers of those turntables would help us evaluate them better.
There are basically two types of magnetic cartridges: standard and P-mount. P-mount was designed by Technics in the late 70s and was used on many of their later turntables including the SL-5. All turntables that used P-mount cartridges had straight (rather than S-shaped) tonearms. A few other manufacturers used P-mount as well. The idea behind P-mount was that the cartridge had a standard mass, size and terminal positions, so no real setup was required. You just plugged the cartridge into the tonearm, added a screw to hold it in place, set the tracking force and you were done. A P-mount cartridge could be converted to standard mount using an adapter (that shipped with many P-mount cartridges of the era), but not vice-versa. P-mount cartridges are still available.
