I've shown a close up of my 1933 and as Richard says I to cannot see just how the alteration was made , but if I were to attempt it, I would cut a 3 off another 1930s penny leaving an extra amount of metal on the bottom of the 3 , Then take the coin that is to be altered and carefully remove the last digit down to a flat surface. Then I would etch down into the coin to the exact shape of the 3 . Finally inset the 3 into the hole with some kind of glue , as I think any kind of soldering would show . Although it would have to be carried out by someone with great skill such as a jewelry maker for it not to show . ???