Hi Karl, A bit off-topic to the question at hand (how to reproduce it), but it is a convenient way to show long rests, and is also how it is done in the manuscripts of the period (so can also be useful if one wants to show it in an incipit, as may be done in scholarly editions, or if one wants to reproduce not only the exact "meaning" but also as much of the layout as possible). Plus it is definitively better looking than just sticking a black bar halfway through with a large number on top (and allows for double-checking in case the part gets smudged or some other accident happens). The practice is clearly inherited from earlier notation, but as far as I know the only rhythmic meaning of the rests is how many measures are to be "tacet", i.e. same as a pause being used to indicate one full measure of silence whether a piece is in 3/8 or in 4/4. I've seen it notated WITHOUT the corresponding number (but with the same meaning as later) in music from the 17th century.
Cheers, Aleksa
