Jim,
Although my original answer to your question was technically correct, I realized it could have been a little more complete.
The rings used on the valves are a one piece, cast iron, L style ring. The "L" refers to the cross section with the long part of the "L" fitting into the grove and the base of the "L" riding against the bore of the valve cage. There are four rings per valve, an exhaust ring and steam ring on each end.
This drawing shows the arrangement of the valve but using "T" style rings.
This drawing shows the different style of rings commonly used. The "L" rings are shown in
(a) at the top.
This design allows for some minor correction to be made by machining the outer section of the ring to different widths to compensate for errors in valve cage port width or spacing, or the valve spool (the center part between the inner rings) being too long or short. It is considered bad practice to make special rings for this purpose due to the fact that, "back in the day", a special ring that was installed by one shop might be replaced with a standard ring in another shop, throwing the engine out of time. In our case, it is moot as the same group of mechanics does all the work in the same place.
Russ