back in the mid 1970's, i built two rifles with surplus actions--it was common to use springfields, mausers, and enfields--i used the 1917 enfield, and the 1909 mauser.
in 1974, i sent an enfield action to ken clark (he was known for his cartridge, the .224 clark--a wildcat using 80, 82, and 85 grain .224 bullets--something well ahead of its time). i only had him do the action work. a real problem with those enfields was the "enfield ears" which had to be ground off. each fellow that did that invariably ended up with a somewhat different receiver contour, and fitting bases, along with deck height issues--made for tough sledding.
these rigs are seldom used as much these days--(slow lock times--calling for speed lock kits, and some, like enfields, needed to be converted to cock-on-opening, etc).
with modern cnc machining, there has been much more uniform results on parts--but unfortunately, in the past year to year and a half or so, i've seen a falloff in quality control with some of the parts i've used--they are not what i remember from just a few years back--and things could get worse. in one particular situation recently, i found the front ring to be lower than the rear--in a case like this i always try another pair. this qc is with respect to the everyday run of the mill components used by the average hunter, and not higher end components used in better, more expensive systems.
in my personal experience, along with the particular firearms i use (no cheap guns, and no surplus military action based rifles), i've found this "height disparity" to be of fairly small consequence. however, when getting ready to mount standard rings, the installer can check for height disparity by using the bar when it is lightly clamped into the front ring after its installation--if the bottom half of the rear ring slides under the bar and on top of the rear base nicely, the height is good. in some cases, the same technique can be used with dual dovetails; temporarily install the front ring, and then with the bar clamped lightly in that ring, move the bottom half of the rear dovetail against the forward side of the rear base--you can just get the edge to it, but it is enough to see if its good. (but remember, for the final mount sequence with dual dovetails, you must first mount the rear ring, and then the front ring). in those situations where you can't check height disparity by this method i've mentioned, if you already have a set of pointed bars, you can also use one of them in conjunction with the scope-tru bar to check for any height disparity. i designed a point on the rear of the bar for 3 reasons--1. so it is easy to install a partially assembled ring on the bar, ie., it acts as a chamfer; 2. to act as a "centering pointer" on the rear bolt plug, action tang, tang screw, or tang groove, to be observed at the final stages of the installation process; and 3. in the event a person already has a set of pointed bars, to use one of them in conjunction with the scope-tru bar to check for height disparity.
if a person is ultra concerned about these qc problems, one solution is that they can be cured with burris posiligns and their eccentric live centers--they're good medicine for same, but i've never cared for the appearance of them. according to paul mcmenamin (accurateshooter.com), he uses them when testing scopes--for when the testing is done, they can be sold "as new", as they have no ring marks.
