Sign post holder is part of an old English lamp post model used for a Christmas parade. It is intended to fit around an ABS pipe and hold a street sign made out of plywood. Check out https://www.thingiverse.com/thing:3211394 and https://www.thingiverse.com/thing:3211293 for the Street Lamp Cap and Flange to make one yourself.

sign-post-holder1.stl is made to hold onto a 3" ABS pipe (this is not the outer diameter but the inner diameter for plumbing). The inner_dia of the Thing is set at 90 mm, depth = 30 mm. It is meant to hold a painted 1/2" plywood board, so the board_width is 14 mm. board_height is 50 mm, hold_length is 50 mm.