Peperomia obtusifolia, baby rubber plant is what I suspect this to be. I can not be certain. I do not work with houseplants. I can barely remember that we studied only a few cultivars of Peperomia that happened to be popular back when we were in school during the late 1980s’. The modern cultivars that are popular now were not even developed back then. I do not remember ever getting acquainted with the simple species, which this seems to be. That makes it even more perplexing. Where did someone get this copy of the simple species that has become so rare relative to popular modern cultivars as well as formerly popular cultivars? The rarest of modern cultivars is not as rare as this original. It is an exemplary specimen, too. It is at least two feet wide and stands more than a foot tall in its wide but shallow pot. Someone has maintained it meticulously for several years. Perhaps it grew as a cutting from an even older specimen that was grown before it became so rare so long ago. Perhaps someone appreciated this original simple species as it was losing popularity to more colorful modern cultivars back then. Realistically, without any of the fancy variegation of modern cultivars, the simple green original is undoubtedly more vigorous, and perhaps significantly so. It is therefore more sustainable than cultivars, and more likely to survive and perpetuate itself by cutting for many years. Alternatively, it could have originated as a reversion from a modern cultivar. Someone may have been impressed enough by its relatively vigorous simple green foliage to grow a copy from a cutting. Perhaps relatively vigorous simple green growth merely overwhelmed and replaced original but less vigorous variegated growth. These are merely guesses about its identity, which remains a mystery.