Below is an early example. Note the rubber stamp endorsement. On some tags this or a similar endorsement was printed on the tag when the tag was printed. I believe this was covered in Linn's. That said it is not a regulation that jumps out in the postal guide. I need to do more work to find the start stop dates.
You will find that such items are endorsed insured and often given a number. However, they, as with this are struck again as registered, or registered round-daters are used to cancel the stamps and track transit.
I like items such as the one shown since during the parcel tax period, the tax still is paid showing this is in fact fourth class not first class, even if handled as registered.
The Insurance paid here was for some amount $50 or less at a fee level of 3/5/10 cents. The higher 25 cents ($100 indeminty) would not support the three cents in parcel tax on postage (not fees and services). This a zone 2 parcel with postage of a nickle for the first pound and one cent for each additional pound. So this tag rates as follows:
3 cents insurance ($5 indemnity), 62 pound package (weight limit for zone 2 was 70 lbs).
5 cents insurance ($5.01-$25 indemnity), 60 pound package.
10 cents insurance ($25.01-$50 indemnity), 54 pound package.