If that really is a modified OEM cylinder than someone put a serious amount of work into it & it looks like they did a great job smoothing & blending every weld they had to of laid down on it. From the center of the cylinder forward it's all filled in & the area below the rear spiggot is also heavily filled in. It actually looks like it's larger diameter at the base too. I wonder if this was an early prototype cylinder for one of the big bore cylinder manufacturers or if it was just a backyard builder with talent. It looks interesting, can you get more pics of it?