I did a search in Google Scholar for "Hill Pits Enclosures Blaenavon", and found a link to the very interesting article written by Laun in the Industrial Archaeology Review, 1979. This can be downloaded for free http://scholar.google.co.uk/.
Regarding enclosures: page 9 states - "Maps (ref 17) prior to the sinking of Hill Pits show the smallholding of Tyr Abraham Harry as two distinct enclosures. The enclosure to the south west (the smaller of the two) is now occupied by part of the tramroad." Regarding the maps, Ref 17 states "Various from 1812 onwards" Just for fun I overlaid the 1st edition OS (1880) map with a Google Earth photo of 2001 (having long shadows). There's a lot of room for errors in doing this - but the the enclosures align quite well on the first image.
A larger area is shown on the second picture. There seems to be an outline of a building or something in the north most enclosure on the 1st edition OS map.