flip ornament symbol

The example I gave is not of a tremolo like you would see in a violin score from the 1800s. In the late 1600s, the double stroke was called a shake, but it might have been called something else when that music for virginal was published a century earlier. The single and double strokes are referred to as graces. At any rate, it's not exactly clear what these symbols mean. Here's are a couple of passages from a recent dissertation on the subject.

1) "If it is placed on a note that needs cadential trills, pianists can use a) upper-note trills with termination, b) trills that start on the main note, and c) turns to grace music. When a note with a double stroke appears after a rest, it should be realized with a grace which begins on the main note.
2) "...it seems that the sign was used as an abbreviation for more than one form of embellishment including an upper note shake and an under-shake.

[See further "William Byrd's My Ladye Nevells Booke: A Guide to Interpretation and Performance", Younkyung Rhea Kim, 2023, pp. 29-30.]

I use the symbol because it's easy to write and also for the sake of uniformity. I don't mind that the symbol is ambiguous because it gives the player some freedom, but I still find it helpful to differentiate between the upper and lower shakes. You might say to just use a mordent symbol for the lower shake, but then I run into a problem if the lower shake is on the middle note of a closed triad. So, I keep the old notation but I use an inverted double slash for these lower shakes. That way, I can use the same symbol throughout the score for the same ornament. When engraved properly, the symbol looks a little different when it's placed on the stem than when it's placed directly on the note head, but the basic form is the same.