I found a nice diagram from an old Dual brochure. With the known mass and compliance of your Pickering system, adding about 1 gram additional mass for the mounting screws, we get:
So, that Pickering system with the Dual 1219 results to 7.4Hz resonance. That's just a teeny-weeny-tiny bit below optimum, according to common doctrine (allowing 8 - 14Hz).
I'd say, that Pickering fits just OK.
With the now known set of values for both the Shure M91 and the Pickering XV15, The "Effective Mass Calculator" of the "Cartridge Database Tools" results in 6.5 - 6.7 grams effective arm mass for the Dual 1219.