Has anyone been able to view the FindMyPast document which refers to Strathallan Castle, or is it the same as the 2001 booklet?
I was looking at the modern pictures last night to see what the objects we noticed actually were, the 'telescope' does appear to be a rock, but I've not been able to find the structure on the far shore. No sign of a Morrison's truck either 
It was a fun exercise!
Nick
I have downloaded the 2001 booklet by J.K. Qualtrough, A Perambulation around Port St. Mary.
There's nothing much to add?!
But I did find, just a little further west, the Perwick Bay Hotel, which had it's own swimming pool.
This hotel would be towards the back right of the original photograph.
My 1935 edition of "In Praise of Manxland" (Maxwell Fraser) offers no insights.
And my 1970 reprint of "The Place-Names of the Isle of Man" (J.J. Kneen) likewise offers very little?
St Thomas' Isle doesn't get a mention.
EDIT:
I found a newspaper report of court proceedings, involving importation of coal without a license.
Approx. 16cwts of coal was removed to "Strathallan Castle",
formerly known as "Point House", Port St Mary.
Another lead?