not sure how i missed that, thanks. I have read Allen’s book."Now, if this Timothy was the renowned companion of Paul, which I am inclined to think, it is very probable that either Luke or Clement was the author of this Epistle." (Calvin, Commentary on Hebrews, 13:23)
Personally, I think there are a lot of compelling reasons to think it could very likely have been Luke, based especially on linguistic indicators such as its particular usage of vocabulary, syntax, literary devices, prologues, and OT citation formulae, as laid out in this book.