OK, I found my copy of G. Schirmer’s “Manual of Style and Usage.” Besides being the house style for G Schirmer throughout at least the last half of the 20th century, it was used by other publishers (as I said above, I got a copy of it from a friend in the NYC office of Peters). It was to apply to computer programs, hand copying, and engraving. From what I can tell, the copy I have was updated in the late 80s or early 90s (Finale and Score were both capable of doing acceptable work by the mid 80s). As far as I know, this style manual is/was not publicly available. It was (still is?) distributed to copyists and engravers.
As with many style manuals, there are things I like and don’t like. Their recommendation for tuplet brackets, for example, specifies that the rh side of the bracket extend only to the edge of the notehead, no matter the rhythm, so 1/8 - 1/4 under a tuplet bracket would NOT cover the rhythmic duration (that style seems to have been supplanted generally). They also suggest that septuplets be normally regarded as 7:8, while it seems more typical these days for them to be 7:4.
Their advice on most things is very fine. And, as a concise 32 page style manual, it makes an excellent and quick read. It often resorts to comments like “an experienced engraver will know how to handle” any number of complex situations, so it doesn’t try to cover everything, but instead seems to encourage people to consult with the “silverbacks” if questions arise.
With regard to trills, their style is this:
Trills should be indicated with the traditional tr above the note. When a note with a trill is tied, a wavy extension line is added [they give an example]. Notice that the extension line only extends to the second notehead; it does not extend for the full duration of the second note. Do not use the extension line if the trilled note is not tied, regardless of its duration [I’m skipping the explanation of the differences between trills and tremolos] …In contemporary music, do not use an accidental above the trill to indicate the ancillary note. Instead, use a small note in parentheses [they give an example]. When a trill is tied onto another line [that is, over a system break], use parentheses: (tr). The extension line is not necessary, unless the note is tied for additional measures.
I follow this practice, more or less. I DO use tr with a sharp or flat above it; I leave it to players to assume that tr without a sharp or flat will be to a natural, but I add the natural if there could be any ambiguity. I use a bit of wavy line, not (tr), to indicate trills continuing over a line break, so that if I change line breaks, I have a minimum of adjustment to do.
As commented on by Romanos401, wavy lines can add to score clutter. I like Schirmer’s style suggestion both because it is unambiguous and is as uncluttered as possible. As I said, I have never had performers handle my trills incorrectly. When a wavy line extends to and completely covers a tied note, it is clear that the trill continues, especially if tr without a wavy line is the default for notes that aren’t continued with ties. Granted, there will be complex and unique situations (like the Beethoven sonata example in a previous post) that require specific solutions.
What I like VERY much about Dorico is that it seems to wish, through user-controllable default settings, to accommodate nearly every standard style choice. Different folks like different looks – which is why in written American English, we see differences between MLA and Chicago style manuals. Gould, I am assuming, is Faber house style. I am hoping only that, for my way of wanting to use wavy lines, that Dorico could allow me (and others who use this style) to have it set this way.