I think it was over-tightened and simply seized up due to age. Front element came off without issue.
I did clean off the Liquid Wrench from the threads, but I'm sure a tiny amount was left. Also, I didn't overly tighten it this time. It was mounted in a Graflex board, it's now in a Linhof board, so probably won't be coming out for the foreseeable future.
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