Thanks for your first answer, as to the fact that this was indeed possible. But making an election for a deferral by year end was something I was unaware of. Both my Solo 401(k) and Roth Solo 401(k) are at Vanguard and when I login, I see I can make a 2020 contribution--but I can't see a place to specify whether it goes to pre-tax or Roth. I presume being able to direct it is what you are referring to about an election by year end. I've had the Solo 401(k) for a while, but last year was the first year with the Roth 401(k), which is what I contributed to in 2019.
