Shimano Ultegra BR-6600 Brake Caliper - $30 (menlo park)

Take-off that had been an earlier upgrade (thus the box). Yes they actually are a bit better than 105-5500 calipers NOTE: This is a single caliper. It was run front, but I don't think there is actually a difference front/back (you verify) I find no distinction on Shimano tech docs. Just flip the pads around for the right orientation.  https://si.shimano.com/en/pdfs/si/8DD0A/SI-8DD0A-001-ENG.pdf https://si.shimano.com/en/pdfs/ev/BR-6600-2368/EV-BR-6600-2368A.pdf