If I recall...
You could put them down on the field in '81, but everything had to be on the field (no pit area on the sidelines). Most everyone marched 4 keyboards (except maybe Garfield? One other who grounded concert instruments?) but had one timpanist grounded. 27 marched the timps, and they were awesome (glad I didn't have to carry them...).
82 saw the creation of the pit area. For a while, instrumentation stayed the same- bells, xylo, vibes, marimba, and a timpanist, with one or two concussionists- for most corps. and then 2v 2m, then...then...then...