The text around the frame reads:
El grande Orpheo / primero inventor
Por quien la vihuela / paresce en el mundo
Si el fue primero / no fue sin segundo
Pues dios es de todos / de todo hazedor.
The great Orpheus, first inventor
Through whom the vihuela appeared in the world
If he was the first, he was not without a second [?]
Since God is creator of everyone and everything.
Ruggiero Chiesa reproduces this image in his edition of El Maestro (Milan, 1974). Strangely, though, the text isn't quite the same. The last line now reads:
Porque es de todos / de todo hazedor.
Since he is creator of everyone and everything.
God has been excised. Worse, this text makes it look as though Orpheus is being credited with being the great creator. Some mistake, surely.
Howard Mayer Brown (Instrumental Music printed before 1600) also gives the Godless text. I would guess that this was printed first, that someone noticed, and insisted it was changed.
The introduction to the recent facsimile edition by the Sociedad de la Vihuela (Córdoba, 2008) lists thirteen surviving copies of the book, but doesn't draw attention to the changed text or suggest that there was any more than a single edition. Quite a mystery.
You'll find a facsimile of the book (with God) on line at the Spanish Biblioteca Nacional.