Hi DB. I recently got the "Sullivan" concerto, but didn't read the notes yet. I'll have to have another go at it with this info.
Yes, It was 'fashionable' in the '70s to refer to Das Lied as Symphony #10 on Mahler collections. Apparently it stopped when somenone began recording the #10 adagio and including it in their release. (Bernstein?)
I think the first complete 10th (Cooke's) was first recorded as an addendum to the Bernstein set, but with Ormandy and the Philadelphia.