There's two vague possible lineages for the Rick Dias, and it's not impossible for it to be a mix of both.

First off is the Dowas, which is generally considered the "final" version of the OYW-era Dom, turned into a next generation grunt MS. According to some lore somewhere, Axis sent some mobile suits on a convoy to deliver a finished model of the Dowas to Anaheim Electronics as part of some secret deal. The Titans find out and attempt to intercept the delivery, but fail. The implication is that the Dowas ends up being dissected by AE and becomes the basis for the further improved Rick Dias.