Could'nt help noticing your post on this site, yes i do know Pengelly croft mine, in fact the field is still called Big Croft.
Your adit is real sure enough, and nothing to do with Wheal Dumpling or any mine to the north. It branches off in there, one adit goes to croft mine, thats only about 80/90 feet to the west on the hump where the field is coloured greyish, the other one (same adit) winds its way southeast to pick Alfred shaft in the corner of the field, then southwest up to Higher Reddalon(where the fir trees are). I do not know what this shaft was called.
The late Lily Cornish's father Dick, once told me he had been down through that adit, he said sometimes you could stand up straight, but mostly on your hands and knees, i think he was very young then, and so was i when i knew him, about 15.
I hope this helps a little, always best to ask someone who knows. :thumbup: