any of the early, printed books were in Greek. Therefore, there was obviously a need for carved Greek initials and we have already seen how, in a pinch, one could turn the letter V upside-down to get a Greek Lambda. There are more examples (not dances of death) here, here, here, here and here, and one could also turn an M 90 degrees to get a Sigma.
These tricks were not sustainable, of course(1) so the printeries quickly acquired themselves a stock of Greek letters, and some of these were decorated with dances of death. British Museum has eight such letters (see external link). The seven of them are simple, caricatured drawings, like the one to the left. The museum does not tell much about the pictures, only: »Attributed to: Hans Holbein the Younger«, which sounds rather unlikely.
The seven pictures are used in the five volumes that Cratander published in 1538 in Basel with the collected works of Galen (I have not found them in other of Cratander's books). The seven Greek letters are mixed with Holbein's genuine letters: H, K, M, N, O, T, X and Y. The Omicron is often used, whereas the Omega and Lambda are only used once each in the five books put together. I assume that this is British Museum's source, since the seven letters that they have, are exactly the same as are found in these five volumes. The Pi was also used twice in Vincentius Opsopoeus' »Diodorou Sikeliotou Istorion Biblia« published anonymously (Johann Oporin?) in Basel 1539.
It would of course be interesting to know the number of letters that the alphabet originally consisted of. Gustav Schneeli reproduces 7 of the initials in the book Initialen von Hans Holbein, table LXXVI, no. XXXVIII, and here he includes a Theta (picture top, right). Unfortunately Schneeli doesn't say where he has found them. The letters are simply a few in a long row, about which we are only told that they are from the 1530ies, but not by Holbein.
The Delta to the right is clearly different from the others (and besides, that series already has a Delta). In fact, the British Museum has two copies of this letter, and the other one is a part of a set of 11 letters (of which none of the others is a dance of death), which the Museum attributes to Jacob Faber.
British Museum writes that these 11 initials by Faber were first used for »Grammaticae institutiones uam tanta adhuc cara excusae«, printed by Curio in Basel, March 5, 1524, but this book does not contain the Delta. I have however, found the delta in »Lexicon graecum iam denuo« (VD16 C 6454) printed by Curio in 1525; twice in »Lexicon graecolatinum«, printed by Johannes (John) Walder in 1537; in »Ptolemaiou Megalês syntaxeôs« from 1538; twice in »Tôn ippiatrikôn Biblia« published by Ioan. Valderum (Walder), 1537; and also in »Lexicon graecolatinum« published by Curio in 1548.
A copy of the same letter was used by Köpfl as early as in 1526.
This could of course just be a few cases of plain sloppiness.