Here is some CGC census info on the early Batman Tecs (copies 6.0 or higher only for unrestored):
Detective Comics #27 - 39 copies graded: 18 unrestored, 21 restored (6 in 6.0 or better)
Detective Comics #28 - 14 copies graded: 5 unrestored, 9 restored (3 in 6.0 or better)
Detective Comics #29 - 26 copies graded: 12 unrestored, 14 restored (3 in 6.0 or better)
Detective Comics #30 - 20 copies graded: 10 unrestored, 10 restored (4 in 6.0 or better)
Detective Comics #31 - 39 copies graded: 21 unrestored, 18 restored (3 in 6.0 or better)
Detective Comics #32 - 21 copies graded: 15 unrestored, 1 qualified, 5 restored (2 in 6.0 or better)
Detective Comics #33 - 44 copies graded: 20 unrestored, 24 restored (4 in 6.0 or better)
Detective Comics #34 - 29 copies graded: 16 unrestored, 13 restored (4 in 6.0 or better)
Detective Comics #35 - 23 copies graded: 9 unrestored, 2 qualified, 12 restored (1 in 6.0 or better)
Detective Comics #36 - 29 copies graded: 10 unrestored, 19 restored (4 in 6.0 or better)
Detective Comics #37 - 21 copies graded: 15 unrestored, 6 restored (5 in 6.0 or
better)
Detective Comics #38 - 51 copies graded: 26 unrestored, 25 restored (8 in 6.0 or better)