Richard Serra’s “Sculptures from 1967 and 1968” is just seven very early works, from before he quite realized the implications of malleability or tonnage. And though these modest process pieces must once have seemed raw, they’ve settled with time into a familiar kind of impoverished elegance. But the show is a gem if you mine it for clues to his amazing 21st-century behemoths. Two lightweight plywood wall works (neither previously exhibited) grapple with relativities of space and surface. A candle piece marks potential time. The twisting spill of ragged rubber, fiberglass, and canvas sandwiched together (it’s handmade) and the double rubber fold anticipate the spatial inversions of his recent megaton spheroids and toroid prows. But the core of Serra’s subsequent work lies in the two works that make the least effort: a small heavy rolled sheet of lead and a warped chunk of vulcanized rubber. Both, in hindsight, hold within themselves unfurled dimensions, like the infinitesimal universes of string theory.