The very narrow cylinders were early. About the 1960s. I bought new ones that were slightly smaller around the port in the early to late 1970s, and the newer ones were solid all the way up, and some had the two slits. I have a few of those but do not know the exact years that they changed, as I bought them used, or they were in boxes with other things. I found they all went about the same pretty much. The single bypass, and twin bypass TD type being the most difference. The crankcases got fatter at about the same times that the cylinders got fatter. I don't believe I have any without the SPI, but think it was likely because of the mufflers. I think shimming the cylinder was only done to make the exhaust ports face forward to get the gasses out. At least with the people I hung around with. I guess it would get a higher power band, and would lose the SPI if it is too high. Then the top step of the liner would need machining to get the compression back.