In modern scientific and technical notation, numerical values are routinely written together with explicit abbreviated unit symbols (for example, 3 ft, 2 mi, 5 lb), forming a single semantic object that combines magnitude and unit.
However, when looking at ancient sources, it seems that numbers and units were often treated separately, with the unit either stated verbally, indicated by a distinct symbol, or simply inferred from context.
My question is specifically not about the origin of units themselves, but about the history of notation:
When did it become common to write numbers together with explicit unit markers as part of the written expression?
In which historical contexts (administrative, commercial, scientific, engineering) did this practice first appear?
Did ancient civilizations (e.g. Egyptian, Mesopotamian, Greek, Roman) explicitly mark units in written records, or were standard units generally implicit?
At what point did abbreviated unit symbols (such as ft, mi, lb) begin to function as stable, conventional elements of numerical notation?
I am particularly interested in sources that discuss this shift from context-dependent units to explicit unit–number notation, especially in relation to the development of early science, engineering, or metrology.