RCBS sent me new agate bearings and I replaced them. The center line will just either center or the lines are either at the top edge of the pointer to the bottom edge of the stationary center line or the visa versa. really close. I weighed some powder today on my RCBS 505 scale and dumped the measured amount into the pan on the 5-10 scale and it centered and measured exactly the same so I guess close is OK. It wouldn't even matter being maybe 1/2 grain difference. Do you think it will matter? I don't load for anything more than having the bullet hit the target close to center, not trying to make one hole with 10 shots.
If your scale is registering +/- the thickness of the zero line and repeating reliably, then I would say you don't have much to worry about unless you are looking for extreme accuracy. Either side of the line is about +/- 10th grain, and as long as you're confident it's reliably repeatable, that's better than most measures will give. If you work up your loads with that scale, what they actually weigh is secondary as long as they are consistent. If you are concerned with accuracy of weight, then checking one uncertified scale against another is not the best way to go. A simple set of grain checkweights from RCBS/Lyman will give you peace of mind.
I've worked on a good many of these simple scales and very rarely find a problem with the agate bearings - they are made of a natural material and I have found, on the very rare occasion, some inclusion or strata line running through one or one slightly miscut off center. It's most unusual to find any wear problems.
The knife edges, on the other hand, are a fairly crudely machined item and very prone to wear and damage, unfortunately, the knife edge is considered an integral part of the beam and will not be supplied as a separate item. At one time, in the event of a problem with the knife edges, RCBS would send out a replacement beam - Now, I understand, they no longer support the older scales, although they probably have a stack of agate bearings, as the same bearing is common throughout the range, 502,505,5-10,10/10, Dillon Eliminator, Lyman M5 etc. with the exception of the RCBS 304 and R130.
This is a handmade knife edge compared to the original example.