That old OH bar will throw more grains of the new powder. It will drop about 19 grains 700X vs. the 17.5 it was designed for. Newer powders are cleaner and more dense. I've got a 0 bar that drops about 21 grains of Red Dot.
MEC sold bars for many years before they finally came out with the powder bushings. As mentioned above, they can be drilled out. I have a couple I've drilled out to accept Lyman powder and shot bushings. I've rarely used them after the MEC bushings became available.