Yank:
the "slide" you refer to is actually called a wedge, and it travels from right to left through a "loop" attached to the bottom rib of the barrels. Eightbore is correct in that is must move further left to disengage this loop and release the forearm.
Try tapping the "slide" gently with a non-marring instrument like a brass punch of correct size. It may move far enough to release. If not, take the gun to a 'smith familiar with such things and have him try. (hint: if he's under 40 years old, don't let him).
The forend tip was likely a wedge of black horn.
Many Scott records are still in the hands of Gallyon's in the UK, but are unavailable for research.