How rare is this?

[D13]

A/C on inlines is not really rare, biggest issue is the swap to integral head and radial compressors which created 2 sets of brakcets for the 194/230/250.
What makes this more rare is 292, the brackets are unique to that engine. You would need the crank pulley which is not real common but still available.
AIR means it came on a later motor. Is that the radial compressor? That would be mid-later 80s which fits with AIR.
Those are a little more common as some of the bigger trucks had A/C. AIR on 250's was usually integral head, but 292's never got the integral.
Still neat and unusual.