I do apologise if this has been discussed somewhere but did a quick search and couldnt find anything.
Reason I ask is have just been going through a few pairs of trs I have and one pair stood out as the others were all ripstop and had green labels,this pair is of heavyish material,has white labels and are dated 1981.
I'm pretty sure 1981 is the earliest date for standard production BDUs. They have always come in two weights; solid cotton and ripstop (various mixes of cotton and poly). The latter are referred to as "Hot Weather" and were supposedly special issue for soldiers operating in warmer climates. In practice it wasn't very strict. DCUs also came in solid cotton and ripstop.
The earlier Woodland items had white labels. I'm not sure when the change to OD labels happened.
By official I mean on any official US documentation,garment label etc as as far as Im concerned and aware it is simply WOODLAND,no such thing as M81 Woodland,the M81 I would hedge my bets as being another collectorism term.