My Excelsior Sonorous flute has a sounding length of 60.5cm so is therefore a low pitched C flute. I’m curious about its value as it is free of splits etc.
Sounding length isn’t the best determinant.
What is the tuning when you play it?
What is the slide extension to get it to A440?
Is it 8-keys, 6-keys or something else?
Is it of wood or ebonite?
I presume this is a Böhm system cylinder-bodied instrument? For those, sounding length is a pretty accurate reference pitch indicator and yes, 600mm is A440. 605mm might be a little flatter, perhaps continental diapason normal A435.
Sharing some photos would help.