wrote:
I gathered that they devoted more effort to the design of the
detonation chamber than to the bombs... or at the very least, more of
their chamber-effort was publicly releasable than their bomb-effort.

I'm trying to remember when enough details emerged publicly about our
nuclear weapons that everyone knew we relied almost entirely on
plutonium rather than uranium for our weapon pits.
Of course there isn't any technical reason you couldn't use U-235 for a
implosion type weapon pit - in fact, that's what Pakistan used in their
nuclear weapons; it's just damned expensive to do compared to using the
same amount of uranium in a breeder reactor to make a far larger mass of
plutonium.
The propulsion units for the Orion generated 1 kt up to 15 kt in the
larger versions; they used 2.9 kg of plutonium each for the smaller
ones. The Orion project did look into ones that used only around 1.8 kg
of plutonium, but that meant more weight in the explosive lens assembly
for the higher compression needed to detonate the smaller pit as well as
generating lower isp per unit; and when all was said and done it wasn't
considered economical versus using the 2.9 kg ones, so one using around
2 kg of U-235 is probably workable.
At the time the Orion report was prepared for NASA (1964) the plutonium
cost $18,000 per kg.
The 15 kt units would have used 6 kg of it.

Pat