I read this entire post. Very interesting. Here's what I conclude from it.
As already known, analogue gauges are subject to conditions that do not allow for ease of corrections.
eg. perfectly level manometer, condition and temperature of the gauge oil, accuracy of zero setting [to a minor degree]
I wouldn't expect any two benches to lay down identical numbers throughout a full lift cycle when testing a cylinder head, even when the same test fixture is used on both benches. Close perhaps, when using the same manometer [digital or analogue] on both benches. The reason being plenum size and shape differences.
A key factor is the influence turbulence has on the manometer probe pickup at any point during the flow test.
Anything that affects the path the air as to take on the way to the measuring point can affect the pressure at the measuring point. That is perhaps the most difficult thing to determine, much less correct. I have said for a long while and I stand by this. A large plenum with as little turbulence influence as possible will yield the most accurate measurement of a pressure differential. Ultimately that's all it comes down to. Measuring the pressure differential accurately
I don't know what the story is behind purchasing the SF600 bench but it's nice to see someone with one that will put forth the comparison effort that ultimately leads to a greater understanding of flow bench construction & design on the overall...
As it is now, there is no real, easily affordable standard to calibrate a bench with except for Bruce's plates. They've proven to be valuable in their accuracy.
JMO
Larry C
. What we are all trying to do here is get to the most accuracy with repeatability. Here at PTS, we are trying to get these variables into a closer "window". We can also squeeze out sharper numbers with the velocity probe. As we do this that window keeps closing. 
