To be certain, you could locate a Metallurical lab that specializes in "Optical Emission Spectroscopy" and let them do an analysis of the frame. It's relatively simple, and can be done in an "out of the way" location on the frame and is relatively non destructive.
I say "relatively" because the area that is tested is exposed to Argon gas while a series of sparks are played over a the area to evaporate a small part of the surface which emits light (photons) and is recorded by an Optical Spectrometer, which, after calibration, provides an elemental analysis of the sample. The small test area has a slight frosted appearance which can easily be polished out or blended.
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