150 series came in both round and square butt versions- usually Service and Security Six's were square, and Speed Sixes round.
Some 151-series Pachmayer grips will work with some minor modifications. My 150 series Speed Six came with Grippers for a 151 series on it. The previous owner drilled a new pin location in the bottom of the grip to match the different 150 series location. It worked, but I wasn't a huge fan of the Grippers and put a stock set of Ruger 150 series grips back on it. Oh, and there were two different pin locations in the 150 series grips, so if you find a set of grips you may have to drill a new pin location- but it's easy.
Herretts grips for the 151 series will work on a 150 series, btw.