Hi 18pounder, It seems you have a couple of options. Yes some units the fins are riveted on and others such as the one I have pictured here there are no rivets. Also hard pressed to see any evidence of a spot welds so how they are attached I am not sure. If you look at the pic you can see that the top edge of the fins closest to projectile is as high as the mounting sleeve goes (yes it is a sleeve). The relic unit I got from smle2009 (thanks Tony) I made the fins from sheet aluminum and epoxied them to the tail to make a 3 fin to display alongside the 4 finner. As this other 'werfer was a relic I did not split hairs on my methods. It is a doable project and time, effort and materials all depend on what you expect when finished. I am very happy with the 3 finner I made, but then again as I mentioned before my expectations were not high. I will show a pic or 2 of the 3 finner I did if you would like to see that. The one pictured here is all original, save the fuze....Dano