For Creedmoor, targets were imported from England and were as used by the NRA(GB). That used at 800-1000 yards was 6ft high by 12ft wide, with a 3ft square bulls-eye in a 6ft square centre, and 3ft side panels as you note, but scoring was 4, 3 and 2. These were the targets used in the 1874 US v Ireland match at Creedmoor.
In 1875 the NRA in Great Britain adopted circular targets and these were the targets then adopted at Creedmoor and used in the 1876 Centennial Match. The target was again 6ft high by 12ft wide. Bull scoring 5 was a 36in circle inside the centre, a 54in circle scoring 4. The 6ft x 6ft inner scored 3, and the remained of the target, the outer, scored 2.
During the 1876 Centennial Match, J.K.Milner of Ireland made the unprecedented score of 75 x 75 at 1000 yards.
No artificial support was permitted during shooting (slings or rests) and US shooters did much to popularise the back position which was found to be more stable than prone.
David