Cf.

https://www.businessenvelopes.com/double-wind...
https://www.businessenvelopes.com/double-wind...
https://www.staples.com/LUX-10-Double-Window-...
https://www.officedepot.com/a/products/679689...

The little bitty line in 7-point type, above the recipient's / mailing address in large type?

I think it would be understandable to call it the 'return address line' or the 'sender's address line.' But for people unfamiliar with German business letters and envelopes, you might need to explain that the cellophane window is on the left, and is large enough to add the sender's address in small print above that of the recipient in larger type.

American practice uses a completely different format, with the return address, if not already printed on the envelope, in its own separate window at top left, and the recipient's address in a centered window about 2/3 down the envelope.