Why would a postal clerk use a blind oval cancel on the front and switch to a date hand cancel to mark the seals on the back?
As per the regulations ... from the 1924 Postal Laws & Regulations volume, see paragraph 3:
Wouldn't they be worth more on a bank tag? Or a post-1932 registered cover?
With very few exceptions, high value stamps are far easier (and less expensive) to find on tags than covers.
All that said, I would agree the pair did not originate on this cover. too bad it has not been retired to the APS reference collection instead of being allowed to circulate in the hobby.