These natural straight edges are often referred to by philatelic professionals (dealers, etc.) as "rarities of the future," since so many here are being reperforated to satisfy collector requirements for fully perforated stamps. I particularly like the one you scanned showing the two straight edges. I like to call these "doubly straight."
Why on earth reperforate this
- it tells an excellent story about how these stamps were printed
108,153,993 million of these 5 cent stamps were issued. There is only one
of these per sheet of 180, and say 1% have survived in collections = there's only some 6,000 of these out there
In fact, given this, it's surprising there are not 'straight edged' ones being made by trimming perforated ones.
Ref - http://www.1847usa.com/ByYear/1912.htm
- Also an interesting story around these stamps - they only had a life of 6 months as specifically parcel post stamps.