Many would have been drilled during late WWII re-builds, after the adoption of the lube/drain hole was introduced on the Type 5 housings.
I'd guess the majority of the other Trigger Housings had the oil lube/drain hole added during the Post WWII re-build programs by Underwood, Stan Pro and the Arsenals.
Lucky that many never had this done.
Someone may know a more precise date. Or dated document as to when this was implemented.
BrianQ comes to mind for these matters.

Regards,
Charlie-painter777
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