Lunar chronology functions are calibrated from Apollo samples that have a well-defined radiometric age and are correlated to a region with a known crater density. Different chronology functions have been constructed based on different crater counts and different curve fits to crater-counting data. The Neukum Production Function (NPF) is the most commonly used chronology function for the Moon, but there also exists the Robbins Production Function (RPF). The RPF is notable for featuring a much more rapid drop off in impactor flux during the Imbrian period. Using an impact bombardment model, we modeled the distribution of impact melt at the Apollo sites for the Imbrian period. Large sub-basin craters like the 250 km Iridum sub-basin may be the source of some melts at Apollo sites, but it is very difficult to determine specific craters of origin for impact melts from this time period. The distribution of Apollo melt ages is more compatible with the steep decline in impactor flux during the Imbrian characterizing the RPF than the more gradual decline of the NPF.