In this paper, I will look at the relation of Armenian and Iranian stem word-finals. Agreeing with Olsen 2005, I argue that the Arm. stem-classes to which the Ir. loanwords are assigned reflect stem and word-final vowels in the Ir. source languages, and the differing treatment of some Ir. stem classes is due to dialectal diversity of the sources. The divergence to be explained chiefly concerns Ir. a-stems, some of which yield Arm. a-stems while others are reflected by Arm. u and o-stems. Evidence from Manichean Middle Persian suggests that am yielded a labial vowel in some dialects of Proto-Western Middle Iranian. I argue that this also applied to the dialect that furnished the unexpected Arm. u/o-stems, and that this dialect is further characterised by a development of Proto-Ir. vocalic *r to ar, thus differing from both Middle Persian and Parthian.