Pinterest won't cooperate with me. The photo gets covered by a "join" screen one second after I open it. Looks like an Underwood 3 to me, and the keys are not original, most likely replaced some time during the '50s.

Well, the shift keys and lock are not depressed as you stated, so that isn't the issue. It's difficult from the angle the side photos were taken, but it could be the carriage is in the raised position. Could you provide a photo/scan of a type sample? Be sure to use every key on the keyboard in order. In the meantime I'll pull out an Underwood 3 and see if I can verify the carriage position in your photos.

Very strange. Doe the carriage move at all when you use the shift key, because from photo 3 and 4 it looks like it does - but that could be an inconsistent camera angle playing tricks on me. The strange thing is that if your carriage was stuck in the shifted position that rod in the centre of your photo should be higher, but as your type sample proves, the machine is only typing the uppercase character set.

The other strange thing is that normally the numeral characters are part of the lowercase character set. A photo of your type slugs would be interesting to look at if its sharp enough to make them out, because I'm starting to wonder if your lower and uppercase characters are reversed. What a fascinating machine you would have if that turns out to be the case. If the carriage is only moving slightly and you can get it to type half characters, then it is an issue with the carriage. I would carefully following the shift key bar on each side and all of its repsective linkages to find what the obstruction might be that is preventing it from being fully raised. I had assumed because it was only typing uppercase that the carriage was stuck in the shifted position, but if you have some bizarre typeset it's more likely that it's stuck in the lowercase position. 

Okay, normal slugs, but still a fascinating character set on that machine with the numbers being moved to the upper case to make room for the French characters (which later would have been done with dead accent keys). It's a shame that the keys are not original. Or maybe the slugs and keys were replaced at the same time. It's all very interesting.

Back to the problem. Your carriage is stuck in the raised position. I assume you "applied pressure" downward on it to get the lower case 'k' in your photo? If so, the advice remains the same as when we started this thread off. Either something is physically blocking the carriage from lowering, or one of its many shift linkages or carriage slides is fouled with rust or something else that is preventing it from lowering. I'd go over everything very carefully to see if you can determine which of the two it might be. When you press down on the carriage, try to determine what is stopping it from going further. Wish I could help more - it would be relatively easy if I was sitting in front of the typewriter - but over cyberspace makes it a bit of a challenge. Good luck!