Does the spade bit #88810 fit in the regular bit extensions #88702 that are not Quick-Change.

No, the 88810 (SPEEDBOR2000's) will not fit into the old extensions, 88702 and 88601. The first SPEEDBOR 2000 bits and the old style SPEEDBOR 88 bits that had a 1/4" shank measured across the corners of the flat. When we introduced the Quick Change bit system on the SPEEDBOR 2000 to fit the quick change extensions 90102, 90106, 90112 we had to change the size of the shank, because it had to be made to an ANSI spec, to 1/4" across the flats. So the SPEEDBOR2000 with the quick change shank has a larger shank thus it will not fit into the old style extensions.