RE: Stock Fin part replacement question
If you have a micrometer or a good vernier caliper (or know where you can borrow one) it's a good idea to measure the OD of the brass barrel nut as there are a number of different sizes.
If you know what the outside diameter (OD) of the brass slug is, the inside threads will always be (for windsurfer fins anyway) either 6 mm x 1.0 mm metric or 1/4-20 UNC (Unified National Coarse threads in the US System).
If you know anyone with a broken off fin, you may be able to get the right barrel nuts out of the broken fin root.
According to the drawings I have the "cross nuts" (barrel nuts) should be 9.0 mm OD (0.3543") in diameter. This is why I recommend a 5/16" (0.312" or 7.94 mm) round drift punch. An 8mm (0.314" dia.) punch would be also good if you can find one.
Hope this helps,