I found more than one-half million Purple Turtles, introduced by Dan Tilque in the August 2004 Word Ways. UX produced the fewest substitutions (41), followed by EX (47). Some are interesting, e.g. ANTHEA/VNTHEA (th is a thorn, in a quotation from c.900), while others contain at least one common English word, e.g. AURA/QURQ (Jabal Qurq Sharaf in Syria), ARENA/ZRENZ (Ayn Zrenz in Tunisia) or AERAE/VERVE (as in Aerae Christi = AD).
"More Purple Turtles (Part 2),"
Word Ways: Vol. 37
, Article 9.
Available at: https://digitalcommons.butler.edu/wordways/vol37/iss4/9