Custom is the model name of the stem, which was manufactured by SR (Sakae Royal).
Symmetric was SunTour's attempt to manufacture a shifting system that automatically adjusted the trim of the front derailleur as the rear derailleur was shifted. I don't think it held any specific location in the SunTour hierachy. They do not appear to have been tied to any particular group. My impression was that they were much like a bar end or Power Shifter, a special design aimed at a niche market segment. It was left up to the bicycle manufacturer to spec them, if they thought it would appeal to their particular target audience.
Due to the interaction of the levers, it necessitated a connecting housing and top mounting. The levers were offered in both braze-on and band mount. This would seem to indicate that they were intended for a wide price range of bicycles and also aftermarket sales.