Memorial to the Royal Regiment of Australian Artillery on Mount Pleasant, overlooking Canberra.
Rifled Muzzle-loading 64-pounder 71 cwt (hundredweight) gun converted from a smoothbore 8-inch 65 cwt shell gun.
I think the markings you're showing on the trunnion face stand for Royal Gun Factory, and the year 1870 would be the date of the conversion from smoothbore to rifle using the Palliser technique. The numbers on top of the tube give the weight of the barrel in hundredweight, 70-3-0, which is 70 x 112 + 3 x 28, or 7,924 pounds.
William Palliser patented 21 ordnance-related inventions, including the armour-piercing Palliser shot. He designed the "Palliser conversion" technique which was used successfully to convert many of Britain's obsolescent but still serviceable smoothbore muzzle-loading guns into more modern rifled muzzle-loaders ("RML") in the late 1860s and the 1870s.
"The 8-inch gun was bored out to 10.5 inches and a new built-up wrought iron inner tube with inner diameter of 6.29 inches was inserted and fastened in place. The gun was then rifled with 3 grooves, with a uniform twist of 1 turn in 40 calibres (i.e. 1 turn in 252 inches), and proof fired. The proof firing also served to expand the new tube slightly and ensure a tight fit in the old iron tube."
