John, Alfred, and All,
A possibility exists that the hole is to accomodate a different type of end mill.
Two Fluted end mills are usually ground in a center cutting configuration and can be plunged at the end if each machining pass to allow cutting on both in pass and out pass thereby saving "no cut returns" to the beginning of each cutting pass.
However Two Flute end mills do not have the cutting feed ability of the Four Fluted End Mill, which would be double that of the Two Flute for the same chip load per tooth.
Main Drawback is that 50 or more years ago four flute End cutting mills were far too expensive for this type of work.
To allow downfeed at the inner end of the groove there would have to be a drilled hole to allow a four flute end mill to be plunged.
This may be a possible answer to the question of "Why the hole?".
This may have been necessary to match production rates of these parts with matching critical parts.
JMO
ViggoG
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