I thought it was designed to hold the center section of a set of panniers.
Back in the dark ages I used to keep 25 feet of 550 cord wound around the rack.
FWIW, my early issue Blackburn rack uses a 'T' bracket like that.
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You know it's going to be a good day when the stem and seatpost come right out.
(looking for a picture and not seeing it? Thank the Photobucket fiasco.PM me and I'll link it up.)