judging solely on the diff that's fitted, that's an early axle, probably 2a. it's not the flat bottom type fitted to most series 3's.
i also imagine as Dr.P says that someone has improved the brakes by fitting either LWB or late SWB Series3 or 1-ton backplates with twin leading shoes, especially if a V8 has been put in. I think 1-ton brake shoes might be wider than norm if that helps, where 'norm' is the width of your rear brake shoes.
there are numbers stamped into the bottom of the axle casing which may help you further, although I don't know who you'd ask ... try the Series 2 club ... (and the numbers still won't help you identify the brakes!)
For further possible ID help I think the diameter of the wheelstuds changed in June 1969 to 9/16", the steering arms also changed at some point, again this won't help identify the brakes though.