Your stamp shows only the top portion of the killer. It is not a precancel (generally meaning a stamp canceled before application to the mail piece), but rather a regular cancel applied after
mailing by the sender. As noted by hy-brasil, the RMS means your stamp was on a letter canceled on a train or at a station, similar to this example, which also shows the entire killer, often referred to a "barred oval":
Here is an older style with the year date outside the town mark. It also shows the curvature of the oval killer designed to match the larger circle of the entire device's base plate:
As you have found, this type of handstamp device comes with a variety of letters and numbers. The town mark and the killer are "duplexed" together as a pair, so you will also see this called a duplex cancel, coming in many types over the past two centuries.
Here are some with letters, likely designating a station. The GC being from Grand Central Station in New York City:
Here are some with numbers, typically designating a device number, or less frequently a station number. Most will be a "1" as their only device.
A use from Warm Springs, Georgia about a month before Franklin Roosevelt's death here.
They are the standard handstamp postmark at intermediate- and large-sized offices. With perhaps a few very rare exceptions, they add nothing to the value of the stamp.