Well - according to wikipedia the AIRCO you suggest closed in 1920 - could be a club of Airco enthusiasts, of course, looking after old aircraft.
A logo search, however, revealed http://www.logosdatabase.com/logo/airco_73259513
: the leading A looks fairly typical, doesn't it? Now - if I interpret this info correctly, we are dealing with an US company: would they commission a weight from Caithness?
Now for the weight: whilst it could be a standard weight with just the base inscription commissioned, it could also be a commissioned design. In the latter case, and assuming Caithness, your chances are dim to find it in the Charlton catalogue: whilst there are quite a few commissioned weights in there, it is only a fraction of the commissioned weights actually made.
I obviously haven't found your weight amongst my Caithness data - otherwise I would have just mentioned the name - but then: I usually spot 1 or 2 "unknown" Caithness weights on ebay each month. Aventurine is not typical for Caithness, however: you mention Aventurine in your second posting - but are likely using it for the material only, not meant to be the weight's name. I have only one Caithness "Aventurine" in my database - which looks completely different
Conclusion: interesting weight - could well be Caithness - but other possibilities should be considered.