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Sorry Dale, I can't help you this time but I'm sure someone else can.
The horn relay is shown in the illustrations book I have for 51 through 56 models, but the 48-50 illustration doesn't show it. On all the later years, the relay is mounted on the firewall above and inboard of the heater hose pass through holes.
I looked at the parts book wiring diagrams and from '51 on the relay shows being mounted on the right side firewall. For '48-'50 it doesn't show where it is mounted, although it is listed on the parts list. On my '49 F1 the relay is mounted up front on the driver side right behind where the wiring harness ends and breaks out for the horn, headlights and running lights. I have a terminal strip mounted there for the light circuits, but suspect it is not stock. My horn is not stock and I suspect the relay is not stock as the parts book indicates it should have a plastic cover and mine is metal. The terminal strip and horn relay are mounted as high as possible on the inner fender. I don't know if this is stock location or not since most of my horn parts apparently aren't original, but I suspect it is.
Johann