Snap Dragon Doubles

Definition 1 
From Granovetter's Conventions at a Glance.

After a competitive auction where the LHO opponent opens, partner 
overcalls and RHO responds a new suit, a double would show the 
4th suit (at least 5) and support for partner (usually 3, but could
be honor doubleton).  Of course, the snap dragon doubler is not 
expected to have many hcp.  

Examples:
  LHO  part  RHO  you
  1H   1S    2C   x    Showing diamonds and spade support

  1S   2C    2D   x    Showing hearts and club support 


  1S   2C    2D   2H   DENIES club support (else snap dragon)

  1S   2C    2D   3C   tends to deny having hearts  


Definition 2. 
From Tiger Bridge, relayed via email from Stephen Li.

According to Tiger Bridge (a book from an English bridge expert,
published in the late sixties) on the convention : Snap Dragon
Double:, it is similar to what you published but quite different
in objective.

In his way of describing :

LHO opened - partner overcalled with a MAJOR SUIT -
RHO bid beyond the TWO level of LHO's opening suit -
my Double is Snap Dragon Double.

Partner passes if LHO bid or rebid his major suit to show
minimum overcalling.

Note the conditions are slightly more restrictive than the one
you described.

It showed 9+ points up with at least game invitational hands
for partner's bid suit.
This way, with limited values in my hand (8+ to 12), it
immediately clarified whether the partnership is competing
for partscore or not while it allows for partnership to
overcall light.

With game forcing hands and FIT (11+ up), my double immediately
assessed the slam potentials when partner may have overcalled
light and still have confidence for slam bidding.

In my opinion, this approach is superior in practice in that
It is important to let partner know how to bid on when fit is
confirmed.

And that the important major suit game propects are immediately
clarified early on when playing against expert opponents (they
can bid vigorously to put us in the dark if fit is not even
confirmed).

And that it is relatively rare to have good results to search
just for the 4th suit as a possible fit when opponents has
opened and responded while we are still bidding with unknown
situation at the two level.