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Symmetric Achromatic Variability (SAV) is a rare form of radio variability inblazars that has been attributed to gravitational millilensing by a ~$10^2 -10^5$ $M_\odot$ mass condensate. Four SAVs have been identified between 1980and 2020 in the long-term radio monitoring data of the blazar PKS 1413+135. Weshow that all four can be fitted with the same, unchanging, gravitational lensmodel. If SAV is due to gravitational millilensing, PKS 1413+135 provides aunique system for studying active galactic nuclei with unprecedentedmicroarcsecond resolution, as well as for studying the nature of the millilensitself. We discuss two possible candidates for the putative millilens: a giantmolecular cloud hosted in the intervening edge-on spiral galaxy, and anundetected dwarf galaxy with a massive black hole. We find a significantdependence of SAV crossing time on frequency, which could indicate a fast shockmoving in a slower underlying flow. We also find tentative evidence for a989-day periodicity in the SAVs, which, if real, makes possible the predictionof future SAVs: the next three windows for possible SAVs begin in August 2022,May 2025, and February 2028.

New Tests of Millilensing in the Blazar PKS 1413+135