hmm, looking down birds eye view again, all I see is a threaded female round hole that takes the short cap bolt.

should the inner female hex allen slot usually be visible and accessible with the cap off? mine is not now. (the BBB users manual for hte BHP21 is not perfectly clear. it looks as though the short bolt threads into the long bolt. But I think that is not the case.)
what do you see with the cap off your extender? allen key slot or a threaded female hole?

I think what I must have done was stupidly whacked the top bolt down, jamming the bit it threads into and obscuring the inner hex slot access.