I would never buy it as a legitimate R3a. In my opinion, while the top and bottoms margins are such that it might possibly be an R3a, the left and right margins are too narrow for us to know for sure... therefore it should be considered as NOT being an R3a.
The margins fall into the area of "we have no way of knowing for certain", so it must be treated as not an R3a. The stamp needs to stand on its own merits, in other words have factors sufficient to preclude it being an altered R3b or R3c.
Also, as Bart mentions, the color (assuming the scan is accurate) is not what I would expect from an R3a. It is too red, indicative of a later printing. In fact, if it were fully perforated, I'd probably be checking the reverse for silk threads...
More than just margins come into play, as the quality control from that era was notoriously poor. Perf/margin freaks abound. See my discussion about an R3b here:
https://goscf.com/t/19117