Source, with rsigns from LH1983, and both options (tone 6 simplex or the proper, more recent, tone)

Assigned only the ancient tone pro defunctis to this I/

Coherence with other 6a/6a2 invitatories

Added NABC + standardization of the "Venite adoremus" part (NABC absent from SG391p257 where other 6a invits are)