If you have the SSS spars that were supplied with most Reedcraft boats, it is possible, when you buy your SeaHawk, that the stainless steel gooseneck assembly will appear "welded" to the aluminium casting at the end of the mast. It shouldn't be like that! You should be able to pull it out from its square seating and then rotate it. Without this facility it is considerably more difficult to roll the sail round the boom for both reefing or stowage purposes.
Try a good soaking in graphited oil, WD40, or other releasing fluid, and then lever the gooseneck out of the casting. Each side of the fitting has a small lip to assist with this. If it's really badly stuck you may need to spend some time gently hammering on a chisel forced under the lip of the side of the gooseneck to shift it. When it finally does come free you may find a bit of white gritty powder comes with it, the aluminium equivalent of rust.
I guess this might be a problem with boats kept on the coast more than inland, especially if wintered in salty conditions with the boom in place.
Note also the cord set into the "spare" hole which secures the split pin. When the sails are set and it is under tension, a slight bend in the middle of the pin is all that is needed to hold it in position. The cord is there so you don't lose it when hoisting the sails and the tension goes. Don't splay the ends of the pin to hold it in position or you won't be able to detach the sail from the boom.
Over time the casting on the forward end of an SSS manufactured boom can soften. This can happen quite suddenly. The effect is most likely to be noticed if the boom is regularly rolled for reefing or stowing the sails. The square end of the gooseneck fitting can gouge out a conical opening and if not repaired reefing would no longer be possible as the square section on the gooseneck fitting would be able to turn inside the rounded hole in the casting on the end of the boom.
Find a local engineering firm to drill out the rivets and make up a new end plate for the boom. The company that repaired this one were full of apologies that the top rivet was not as neatly fitted as the original. They had had difficulties in drilling out the old rivet. However, there were no complaints from the owner, as the £25 charge was certainly a lot less than it would have cost to source an acceptable alternative boom - and it looks as if it should last at least another 40 years!