For a lock that's 50-60 years old, I'd be tempted to replace the whole thing. Just go to the hardware store and pick a similar one up so all you need to do is unscrew the old and screw in the new. Typically the inside of the lock is screwed into the outside half of the lock, and the bolt is screwed into the door on the side.
The biggest gotcha is if the hole changes size or position, not uncommon with an old lock, which is why you're better off finding a very close match. When I had to enlarge a hole in a metal door, I managed to rig two hole saws together on a common center mount so that the smaller hole saw guided the larger saw. Without this particular hole saw model, or if you need to shift the position of the hole, you'd need to first patch the hole so the saw has something to keep it from sliding all over the door, and of course this only works if you're enlarging the hole to include the entire space of the old lock.
Rekey
You can probably rekey the locks yourself, depending on what brand and type of locks they are. The process isn't too difficult, you'll be a pro after one or two.
You'll have to remove the handles and deadbolts from the doors, so you'll probably want to go one at a time. But if you like doing little projects like this, rekeying them yourself could save you some money.
Check amazon for rekey kits, but make sure you know what brand locks you'll be working on first. I recently picked up a kit for less than $20. If I remember correctly, the kit had 50 of each pin and all the tools that were required.
If you decide to go this route, there are tons of great videos on YouTube that explain exactly how to do it. Watch a few, and you'll be an expert in no time.
Locksmith
A locksmith will probably save you a bit over a wholesale change out. Again you'll probably want to do it in stages, unless the locksmith can guarantee they will be done in a couple hours.
Replacement
Replacing all the hardware is probably going to be the most expensive option. However, it does give you the opportunity to switch brands, styles, quality, security level, and technology level.
Depending on how many doors you have, you might still have to rekey some of the new hardware (or special order it). Since most packages off the shelf only handle up to two doors.
Best Answer
Take that last photo to a locksmith to be sure, but my money's on a Kwikset-compatible keyway
The last photo you provide (head-on shot of the outside, with the keyway clearly visible) tells me that whoever actually made the lock is probably irrelevant -- the keyway strongly resembles one of the de facto standard Kwikset keyways that are used on low-end door locks across the USA. (It can't be a Schlage due to the "squared" shape of the upper warding piece, and it can't be a Yale either because of the lack of a "tail" at the top of the keyway. The warding is also not nearly paracentric (across the middle of the keyway) enough to be found on a higher-end lock -- even Schlages are a fair bit this way, never mind an Abus or some other European make.)
Of course, IANAL (I am not a locksmith), so if you want to be sure, take that last photo and your keys to your friendly local locksmith and they'll be able to tell you far more confidently than I can what you've got. They almost certainly will be able to rekey a Kwikset-compatible lock as well, as they're compatible with most "generic" lock pins out there.