@mike_shupp Thanks for your reply, and sorry for taking so long to get back to you. @Vistapocalypse gave me a good tip that maybe kb4338380 won't install due to earlier Meltdown and Spectre patches I'd skipped (they broke Sandboxie on my system).
Gave them another try with an updated Sandboxie (v5.26), and here's where I'm at now:
March 2018
KB4089229, KB4090450
These were the original updates that broke Sandboxie (SB). Now installed and working with SB 5.26.
April 2018
KB4093478 This one also broke SB, but has since been superseded by updates in Jun,Aug 2018 - installed fine, SB working.
July 2018
KB4340583 Never tested this one against SB, but it was superseded by KB4341832 in Aug - installed, and SB working.
Aug 2018
KB4338380 Unfortunately, still getting - "This update does not apply to your system". I'm not overly worried atm though, it's currently marked by MS as "Exploitation Less Likely". I'll give it another look next month.
On the upside, I'm now fully patched up to Aug, minus KB4338380, which might very well be replaced down the road. Kinda more pleased right now with the Sandboxie fiasco fixed.
Thanks @mike_shupp , and @Vistapocalypse for your help.