The donut shape sealing ring between the pipe flange and the iron manifold will often have a metal cover on one side and uncovered fiber surface on the other side. This begs the question, which side up? I don't suppose it makes much difference either way. I wouldn't get into any argument about it. John Twist recommends mating the fiber side to the iron manifold and the metal side against the pipe flange (but I still don't know why). Some other cars (notably transverse engine front wheel drive) have a flexible joint here, in which case the metal side goes against the exhaust pipe to tolerate the motion. Perhaps that is the only reason the donut seal has a metal side (for those other applications).

Original issue donut seal measures 1.767" ID (give or take a couple thou) so it will slip easily over a 1-3/4" tube. Original pipe diameter is 1-3/4" OD tubing. It is expanded to 1-3/4" ID for a short distance before forming the flare on the end. Another short piece of 1-3/4" OD tube is inserted into the end of the pipe to serve as a pilot tube extending about equal to thickness of the donut seal ring. When the joint is bolted together, conical surfaces on the pipe flange and the iron manifold flange will compress the seal ring and make it constrict onto the pilot tube. The seal is then captured between two cones and the pilot tube so it cannot blow out or crush inward.

The seal ring is a generic part that will cross reference to different manufacturers and will fit a lot of different cars. Some alternate part numbers follow:

AHH5146 Packing--exhaust pipe
BMC original part number
Gasket, exhaust flange, MGA,MGB,MGC
1-3/4 ID, 2-1/4 OD, 1/2" Thick

1-760   Victoria British - Seal ring (1-3/4")
470-230  Moss Motors - Seal ring (1-3/4")

JF040 ACL - Exhaust Flange Gasket - ID=51mm, OD=60.5mm --Note this one is oversize for a 2 inch exhaust system).