No. There is no R68b. This isn't a "part perf", it's just a corner sheet stamp. Part perfs are the revenue equivalent of a coil stamp. So either both sides or top and bottom need to be perfed. But a side and a top/bottom is just from a corner.
Also, even if it does have parallel imperforate sides, there are numerous factors that need to be considered when determining if it is genuine (size of margins, cancel date, cancel type, ink color and impression, paper color and type), and EVEN THEN, when it comes to the scarce part perfs, you need to get it expertized.
The most insidious fakes of part perfs are not those that are created by trimming perforated stamps, but rather those created by adding perforations to imperforate stamps, which have the correct paper, color, and impression types. Both part perfs and imperfs share the same characteristics of early printings.