Celestial bodies do not have uniform densities and the usual method to deal with this fact is to model the gravity potential field using Spherical Harmonics.
However for bodies which have large non uniformities in density (such as the “mass concentrations” on the Moon), the spherical harmonics model needs to have high degree and order to be useful (sufficiently accurate). At the time of Appollo mission, it was certainly not possible to implement high degree and order spherical harmonics on a computer (and not enough observation time to determine the 1000s of coefficients). However if you can figure out where the mass concentrations are, you can also build a “mascon” model which is just a set of point- or uniform disk shaped- masses that can model the mass concentrations. The best approach is probably to build a spherical harmonics model to modest degree and order and add a mascon model on top to adjust for the high spatial frequency variations.