Horton Safari is the closest, but the original is reportedly by Gunmark.
From Bernard Horton (posted by GotWookiee):
The original bowcaster was based on "a skeleton stocked crossbow designed and made by me for the 'SinglePoint' Company in the UK."
This company developed and manufactured the first red dot sight, invented by a Swede, Neils Ruder, I believe his name was, in the early '70s.
It was expensive and well made and aimed at the military market. They approached me at that time to develop a military style crossbow to take their new sight and which could be used to demonstrate the speed and efficiency of the new technology sighting system.
The bow was made in the same way as the Monarch Supreme; die cast aluminum and nylon coated.
Probably 12 to 15 thousand of these crossbow were sold during the early and mid seventies, mostly in the UK. In the latter years they were sold to the civilian market by an off shoot of the 'SinglePoint' Company.
I imagine very few were actually sold in the US.
I hope this answers your questions on the origin of the bowcaster, I am sorry it has taken so long to get to the eventual facts.
A little more info for you. The Company that took over the selling of the crossbow was called 'Gunmark' Based in Winchester. The owner was one of the Directors of SinglePoint, his name was Bjorn Waktare. I believe the crossbow was simply sold as the '
Gunmark Crossbow'.
I trust this is of some use to you.
Regards Bernard Horton.
----------------
That's pretty much all the known info.
The Horton Safari seems to be a commercial plastic version of the aluminum crossbow Mr. Horton designed for Gunmark.
There are several god pics in the bowcaster thread.
Especially this page:
http://www.therpf.com/showthread.php?t=872&page=17
BD - The one in the ad you found seems to be a modern derivative of the Safari.