Probably not an issue on a '55, I think? Hole will be there?
I'm not sure the crankcase flapper breather is that effective tbh, esp at revs (as Mick hints)
No hole....Breather - surely mainly a function of the spring pressure exerted on the valve? On the breather it seems far less than on the PRV - however, it may not be adequately sized for the job, thereby allowing pressure to build?
Perhaps as my bike is - allegedly- newly rebuilt, crankcase pressure is a tad high. As the piston rings bed in, maybe the problem will go away.....