This gall corresponds to Russo (2021) Procecidochares sp. B described on p. 286 of his "Plant Galls of the Western United States" and has not yet been added to the Gallformers Database which may be why so many iNat examples are being misidentified including some of my own observations when I was too lazy to open the book or didn't have access to it and just used the online Gallformers site in its place.
"Gray-green and densely covered with white hairs. These galls have shorter bracts [than sp. A] and usually occur only on the terminal buds of Rubber Rabbitbrush." The description continues... and matches very well except for the phenology which may have a California bias.

Russos name for this undescribed Procecidochares species: Hairy-Bud-Gall Tephritid

No distinct odor. Cups 1.2 - 1.4 cm across. Stalk 1.3 - 1.9 cm long x 1.5 - 2 mm wide. Spores (19.3) 19.5 - 23.1 (24.8) x (8.6) 9.2 - 10.2 (10.3), Q = 2 - 2.5 (2.6), N = 8, Me = 21.8 x 9.6 µm, Qe = 2.3.

A few florets in the bloom of a Sambucus cerulea had slightly swollen ovaries beneath them. I opened one from one shrub and one from a few yards away.

I was expecting Elderberry Gall Midge (Schizomyia umbellicola), which I have twice found in the county, one of them < 10 miles away. The midge produces much more swollen ovaries--particularly the June 27 one, which had swollen, very round galls. The May 11 one was shaped more like a vase.

Is this unknown larva a gall inducer? IDK. Maybe.

Gall on Cascade aster stem
Measures 1cm long by 1/2cm diameter
Monothalamous
All frass concentrated in the apical end of the chamber
Larvae had head capsule (moth or sawfly?)
Will attempt to rear adults
Found on several plants nearby

The plant continues normal growth above these galls, often flowering