I have early 71 demon one J 915 one X 894 All original car bought it from 1st owner. I will install matching J 915's. Have seen 70's with X's , O's and Z's 894's and 915's combined. There are U's that look to be J's do to being cut off by the plug hole. I hae here one set of 915, 188 valve and 915 202 valve they have different port size and ramp. 188 is a lot smaller bowl's. and some that are same. Heads are not always matching sets so beware of what you are buying.