I saw the macpelah plan and spoke to Cymdeithas Teuluoedd Gwynedd who created it. It seems that their plan does not correspond with the plot numbers used by ‘y ffôr.com’.
There seems to be another plan somewhere. I sent an email to llanllyfni parish council, but they didn’t respond. Apparently they hold the official plan. Some of the plot numbers are on the back of the headstones.