New to Typophile? Accounts are free, and easy to set up.
The forums have occasionally touched on the rules for using a long-s or an eszet, but I don't recall having seen a treatment of the rules -- or lack thereof -- for the archaic "r" in such blackletter texts as the Great Bible.
I came across this glyph when I sought out Cranmer's Preface to the Great Bible, wherein the more familiar "r" glyph and the less familiar one seem to be mingled without a predictable rationale. Have my powers of inference failed me, or did typesetters use whatever came out of the font?