Digging in the OO Inventory I spotted again this car which is an interesting one, an ATSF express reefer.
Looking online I also discovered that the model is fairly prototypical in look! Compare the model to this prototype photo linked here.
I touched up paint dings on the body a bit too and replaced a missing ladder. The car still looks a little rough (the finish is not as splotchy under normal light) but I like how it came out and that it has the ATSF mystery maker sides. The trucks I should also note, I am not sure what they are but the side frames are vintage HO parts to which I added Ultimate wheelsets. They resemble the ones in the prototype photo fairly closely. All in all an enjoyable and fairly quick project, this car will see some use again when I run vintage items.
UPDATE: I am fairly sure this is actually a Unique Miniatures OO gauge car. I missed this in the OO series as they don't seem to have actually advertised. However, in my older checklists developed roughly 15 years ago I have a listing for a 1941-42 product by Unique Miniatures of California, a 45' ATSF express reefer. This model fits the bill perfectly. The mystery for me now is how did I have that listed on that checklist? I must have received a tip from someone about this specific, unique model.