I have this set. Unfortunately when my late father bought it from a childhood friend about 30-35 years ago, I stuck them in a traditional photo album and pretty much ruined the backs. Dumb teenager. I remember they were stunningly near mint when we got them
but back in the day plastic sheets for standard cards were a new concept, let alone anything for oversized stuff.
I have all Tigers/Cards and yes, there are 12. I would guess the envelope maybe adds another $100 to the value? I have the R309-1 Goudey Premium set with the original envelope (bought from the same source).
Never heard of another set being issued so that would be an interesting read.