I also suspect that the term "straddled", as conversationally employed back then, could also include in its range of meaning what we nowadays define as "bracketed".
In gunner's terminology back then, as now, bracketing a target is landing a shot over and a shot short in different ranging shots or salvos. As an example, lets say range estimate for a target is 15,000m. As gunnery officer I order a 500m braket, which would consist of one salvo on estimated range and one salvo 500m over estimated range. The chances are neither will be on for range, but seeing where they fall will help me make corrections. If both are over, I order down ladder by 500m, and order another bracket until I get one salvo landing over and one short. Once the target is bracketed, I will know the range, and can order fire for effect and keep the subsequent salvos on target by making minor corrections based on spotting.
A straddle in gunner's terminology is shells from a single salvo landing both over and short of the target. This is considered ideal, since if one is firing more than two guns in a salvo, there is a good chance of scoring a hit.
is true. I'm still seeking a satisfactory explanation of a "zig-zag" salvo sequence.
Without knowing exactly how the term zig-zag salvo sequence is used as referenced, I can offer a guess. In salvos fired electrically from the director or from the transmitting station, the tendency would be for the shells to land in a line--true there would be a bit of dispersion from gun to gun so that the line is not exactly straight, but if the grouping is quite close, spreading the line in a zig-zag pattern can actually help in achieving straddles. It is rather like using a shot gun on birds--too tight a pattern will result in more misses than hits since a bird in flight might not find itself in the shot pattern at all. Spread the shot out a bit and your have a better chance, as long as the pattern is not too wide.