Assault Rifle, early 1928.
"A" stands for the first design of the part, and "R" means it must be kept for repairs, as the next design of the part that replaces it can't be fitted correctly to the earlier models.
Early wheels are a common part that many use to determine what they refer to as an "AR" model. The early brake system without the separate parking brake used wheels with a narrow hub, and these can't be properly exchanged with the wider hub later wheels.
The part could also be a "BR", "CR", "DR", etc. depending on how many design changes were made to the part.