The rules and descriptions of dating precancels evolved a bit between the original announcements in the Postal Bulletins of March 15 and 22, 1938 (PB#17396 and 17401) and the Bulletins of June 8 and 28, 1938 (PB#17456 and 17470).
The cut-off is "above 6 cents", rather than 5 cents as noted above. Here is how the wording came out in the July 1, 1939 annual Postal Guide:

And here is how it appeared in the July 1, 1947 Postal Guide, which explains why there are denominations of 6 cents and below with dates and initials:

Some collectors seek every combination of user/month on every denomination used.