it is 2018. if the person has put away into the after tax for 35 years, then that goes back to 1983.
A Roth contribution wasn't even possible
"Under a Roth IRA, first enacted in 1998,....." (and for qualified plans it was 2006)
so it would be impossible for the after tax contributions to have been a ROTH.
I think QDROphile hits it. You can always roll a distribution from a qualified plan to a Roth IRA. The NEXT question is how much of it is taxable when that happens.
Sure, use 541990. But recognize also that nobody cares and there is no penalty for using the wrong code. Just wanted to point out that this is a trivial issue; we try to get it right but we don't effort over it.
FWIW.