The tang was changed at the sametime as the safety and like the safety you will find a lot of variences in dates and tangs. In 48 they would GENERALLY be the pre-war style and 49 they would GENERALLY be post-war but I've seen 49's with pre-war tangs. Mostly in the odder calibers such as 22 Hornet.
If you don't already own a copy of Rule's Rifleman's Rifle I would highly recommend that you pick a copy up. You can now get soft covered copies for under $100 and it can easilly pay for itself in just one M70 purchase, or not buying one M70 because it's wrong.