thanks for ref - not to hand at present but will look at it in museum; however I am surprised that Jenkinson in his 1874 guide (a generally excellent and well researched guide in many respects) makes no mention of such a tourist object and tale - he describes walks around Baldwin valleys, Keppel gate etc but makes no mention of the wall picture or story yet he obviously both walked extensively and spoke to many as he notes other objects.
Arguments from silence I know are always a problem but it is surprising non the less.

Oswald mentions many boundary cairns - including the main one at Gerjohl - I wonder if this was another such, as the Onchan/Braddan parish boundary turns sharply along the river