NOTATIO

If you could post a more complete example (including the viol) the meaning of this symbol might become clearer. Another possibility occurred to me from what you said in your last post.

And happy birthday no 76, David!

John Ruggero wrote: ↑21 Feb 2017, 23:14… …And happy birthday no 76, David!

Thank you (due on Sunday).

Other possibilities:

It's a cue so that the keyboard could fill in for a missing instrument?
The note actually defined by the custos is to be ornamented in some special way?
The custos has different meanings depending on the situation?

"Notes to be plaied when the Master of the Viols comes not to Evensong, having a soare heade from the tavern."

Yes, that's the most likely. I've asked a few friends who specialise in playing early keyboard stuff. That should sort out whether it's a well known device for the period, or if it's idiomatic of the scribe.

You might put that as an footnote in explanation!

Yes, it would be interesting to know about this other life of the custos, which matches up well with its better-known career.