Thanks, David - The 909A is the only part on hand now, and I'll keep eye out for 909F going forward. Thank you for clarification on the 909 series properties - that is helpful!
If I were you I would check the specification of the 909C vs 909F. The 2 GHz 909C is likely to be cheaper than a 909F, and I rather suspect that it will have a better specification below 2 GHz.
As my accuracy expectations are not high (eventually I think the limiting factor will be the BNC connectors themselves), hopefully my 909A substitution for the 85031B load will allow me to proceed trying to identify properties for the UG491/UG914 BNC in-series adapters. With a work flow established for this procedure, it could then be repeated once a 909F or whole 85031B kit is available.
Or look for parts from an 85050B, C or D. Those kits often come up cheap and include extras like a torque wrench and connector guage. There’s an incomplete 85050B on eBay at $420 or offer, but I have bought a more, but not fully complete 85050B for half of that.
A 85050B is in my opinion more suitable for most people than the more expensive precision 85050C.