In researching Orussus, I found
The Insect Community of Dead and Dying Douglas-Fir 1. The Hymenoptera, 1975, by Mark Deyrup.
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The key items quoted below lead me to believe that this specimen might be occidentalis. The notion that Orussus uses old beetle holes fits with my very brief observations on the specimens I photographed. All were on very old beach logs.

Here are a couple of excerpts:
"Cooper (1953) made a thorough study of the anatomy and
behavior of two eastern species of Orussus. The two oviposition sites
examined both proved to be abandoned buprestid galleries. The female
appeared to discover these galleries by tapping the wood with her abdomen.
While not discounting the possibility that Orussus is parasitic, Cooper
presents an alternative theory: the larva of Orussus lives on wood par-
ticles and fungi in the galleries of buprestids and follows the galleries out to the pupal cell, where the parasite perhaps attacks the beetle
larva if it is present. This second theory fits in well with the habits
of other members of the Siricoidea."

"1. Thorax ferruginous; distinct sharp ridge present
behind eye; length 5 mm . . . . . . . . . . . . . . . thoracicus
1'. Thorax black; no distinct sharp ridge present
behind eye; length 10 mm or more . . . . . . . . . . . occidentalis"

What a beautiful animal.