Unless it has some kind of identifying logo and serial number on it, or is an original numbered GM part, I don't think there is any physical way of determining the stall spec from the outside.
An expert converter builder might be able to ballpark it by opening it up.
That being said, I think a stock powerglide converter on a V8 was around 1100. Higher if it was behind a 6.
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65 Laurentian post, 67 Grande Parisienne 4 door HT.
