The 8A part number will fit the Fords and later 52/53 Mercury but I don't know if it will fit the 49 through 51 Mercury. The 49/51 Mercury cars had it's own part number 8M-5780-A. The length or pin diameter may be different. The 1949/51 Mercury also has its own bushings by part number.
General spring shows shackle kits for 49/51 Merc but no photo. Jamco Suspension sells the rear spring front single pin kit. They do a lot of custom stuff for Mercs.
https://www.jamcosuspension.com/prod...wbolt-flb4951m
FCRC machine also sells them and may be the manufacturer.
http://fcrcmachine.com/rare-parts-re...mercury-35590/