What does XRO mean?

[Ric Gillespie]

Is it on the inside or outside face, where we might also find traces of it in an existing image?

Inside face. Nothing visible to the naked eye but it's reportedly under some cool growth.

The letters are a little over 1/4 inch tall. Seems too small to be some kind of shop marking, but - FWIW - it's the same size same as jotted down notes I take.

[Matt Revington]

When I look at the letters in the image they could also be XRD.  This is a commonly used short form for x-ray diffraction in labs.wWs this piece ever sent out for X-ray -difraction testing previously?  While I doubt any responsible lab would write directly on an artefact could the markings be scratch-through if someone wrote on label attached the sample while it was in the lab?

[Ric Gillespie]

When I look at the letters in the image they could also be XRD.  This is a commonly used short form for x-ray diffraction in labs.wWs this piece ever sent out for X-ray -difraction testing previously?

The NTSB lab did Energy Dispersive X-ray Analysis (EDXA) on a sample cut from the artifact.