Yup, Harmonic Damper Weights.
My 96 D1 has the round heavy weight on the front diff housing and those long weights on the swivels. My 97 D90 has the long weights on the swivels but not the round weight on the diff. My 98 D1 has both the swivel and the diff weights again. So I guess some 97 D90s had them on the swivels and some didn't.
Glad the D90 doesn't have the diff weight as they are really, really easy to knock off.......
Early/Mid 80s and early 90s Euro RRCs didn't have either, nor did mid 80s Euro D90s. At least not the ones I have owned.