When navies shared signal flags and when they did not

Did navies historically use the same signal flags or proprietary systems?

Both occurred; many flags were similar, but some navies used their own systems before international standardization.

Many navies converged on similar signal flags while some retained their own systems. The internationally recognized scheme familiar today did not exist unchanged across all periods of naval history. A reader therefore has to date a flag image and identify the navy before assuming a modern meaning. Even the visual alphabet behind a signal could vary by navy and period.