The AR is simply the earliest of Model As. Model A aficionados typically debate what serial numbers are true “AR” models. Typical to Henry Ford there are even some leftover Model T parts found on the earliest Model As. The easiest way to spot an “AR” is the generator, steering wheel and parking brake lever. On the early Model As the generator is known as the “Powerhouse” generator (it is shorter and larger in diameter than a typical generator), the steering wheel is red versus black, and the most obvious is the parking brake lever is not on the transmission (near the shift lever) but near the driver’s side kick panel.
Generators and steering wheels were commonly replaced with the later style and more plentiful parts. But, relocating the parking brake lever takes more effort so that is the most sure thing to spot an “AR”.
You are correct that the HVT do not support these pre-war models. However, no price guide is anything more than that so a little market research should establish a general range. But as we all know if a car is truly spectacular, or a rare example of the model, or has been shown at Pebble Beach etc its value will be some multiple of “average” examples.
I hope this helps, and best of luck!