Need the rectangular part

You weren't kidding, this is some old window. What's difficult about identifying your lock is that the sweep part of it seems to rivet directly to the window frame itself. I can tell you with confidence that very few locks work like that these days. I don't think you'll be able to find this anywhere.

Most locks we consider "old" rivet onto a base first, which then screws to the window frame. I say "old" because these are still more modern than what you have here. See our 90-021 for example. You may be able to get something like this to work on your window but there are two issues you have to consider.

1. It may not fit in your old keeper. We have keepers that are compatible, like the 90-108 or 90-114, but that brings us to the second issue, which is...
2. Drilling new holes into the window frame carries the risk of hitting the glass and shattering it. If you want to try to use one of these "newer" locks as a substitute, this is something you have to be very mindful of. Double, triple, and quadruple check your measurements, and if you have any doubts, don't push your luck. 

If you're handy, then I think you could get a lock like this to work on your window frame. If you aren't sure, though, then my recommendation would be to get a local professional to come out and do it for you.