R7b doesn't exist. Best case scenario, this was done privately, not officially. Certainly the nature of the perforations indicates it was not done on a normal perforator. I thought it was interesting enough to put in a snipe and won it for a nominal bid.
It's an early cancel, and during this period some companies were looking for ways to make imperfs easier to use. See, for example, the sewing machine perforations on R19a and the privately rouletted 1st issues (only a handful of examples reported).
Part perfs in general have a very definite answer. The hurry up and ship order was to ship whatever was available at that time, either unfinished (imperfs), or half finished (part perfs). The part perfs exist for this reason alone, time did not permit large quantities of stamps to sit around waiting for the machines to be adjusted. But a "private perf" creator would have no reason not to take the extra 30 seconds to do both directions, since they would only be doing a very small quantity.
I suppose it could be a collector-or dealer-created item, but to what end? Those perfs wouldn't fool anyone, and while R7c does catalog higher than R7a, it's not enough of a disparity to make that effort worthwhile.
Just some ramdom thoughts here...... The perf's do look like pinking shears did the cutting, given the shape. The detail is way off. So, in my mind anyway, it is either made to fool collectors, or a forgery used to pay the two cents. But, if it is to avoid revenue, it would seem we would of seen more of these. Awful lot of trouble, time, and effort for a tax dodge. Interesting nonetheless. The reasons are probably lost to history.
Possibly torn along a sharpened, toothed edge - like a saw blade. If you look at a tape dispenser, you tear the tape against a plate that has little saw teeth. This could be the same, but with wider teeth.
Cut the sheet into strips and then "dispense" by tearing along blade.