The OE hose is 7.9mm in diameter, so yeah pretty dumb close to 5/16ths.
The tube from the fuel filter is actually a threaded connection which leads to a clamped connection of the hose to the fuel rail.
The female portion of that connection is here:
Fuel Rail Supply Fitting (Fiat Bertone X1/9, 124 Spider 1980-On w/FI) - OE / RENEWED
www.midwest-bayless.com
The hard line leading to it is this:
Fuel Line Fuel Pipe Metal Line - SUPPLY (Fiat Bertone X1/9 1980-88 w/FI) - U8
www.midwest-bayless.com
Looks a bit like the brake/clutch line compression fitting
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