When did Victor cease matrix numbers in the record run out?

[Governor Flyball]

From Victor beginning the matrix numbering system which continued from 1903 to 1942, sometime in 1906 the hand written matrix number moved from the groove run out area to the center of the disc. After 1906, the matrix number was concealed in US pressings because the label area was recessed. UK and Canadian pressings they were sometimes partially legible because the labels were not recessed.

I have been curious to find exactly when it was done. This was brought to my attention today when on record 35008-B, earlier issued as catalog number 31480, recorded September 1906, the matrix number is not present on the run out.

Has anyone more information on this?