What are front left foot numbers on an Underwood 5 ?

The Underwood 5 typewriter has the serial number stamped on the frame-top near the right spool. The number itself is generally followed by a type-designation, so the pattern is xxxxxx-5 for a regular Nr. 5. The number is on a flat, machined surface and linear - feasible to stamp with an automatic numbering stamp.

Here the Underwood 5 serial number on a 1920 machine (type "5" on the extra face/facet):

Probably on every Underwood 5 (or 4, 3, 46T etc.) there is another number stamped into the frame. This number is stamped in the flat, machined surface under the front left foot. The digits are arranged around the screw-hole for the foot, to fit in the area. This makes it less likely to have been an automatically incrementing numbering stamp, but more likely hand-stamped. 

Here the extra "front left foot number" on that same 1920 machine:

Another example; here the front-left foot and the top-right of the frame in one picture:

(Yes, it's in pieces. This Underwood was completely broken; dropped/tumbled onto a concrete floor from some height. The cast iron frame was shattered and all panels crumpled. The wreck is being salvaged for parts to fix other machines.)

From these and a few other machines, e.g. on The Database or mentioned on the net, some serial numbers with their front left foot numbers can be found. 

  serial number    year    front left foot number

        475,051    1912          541,824

        734,809    1914          874,286

      1,411,885    1920        1,812,200

      1,872,237    1924       24,611,255

      2,446,801    1928          389,092

      3,677,613    1930          111,938           (46T)

Can't help but wonder; what are these numbers? 

They don't look to contain dates. They do not correlate with the machine serial numbers. Come to that; are they actually serialised, another serial number range for the foundry? They are not all incrementing. 

Are they perhaps job-numbers; per order of a batch of finished-frames so that several machines within a range of serial numbers will have the same front left foot number?

Could the "24" on that 1924 machine be a year prefix to a 6-digit job-number? But that wouldn't explain the 7-digit number on the 1920 machine (late 1920, November-ish).

Curious.