Sean- Im not too sure about the dates, but after the 49 Open the course lost its appeal a bit and got dumped off the Open rota, partly beause of access and a bit because of the blindness of some hole. I can only presume the club made a number of changes as part of 'the plan' to get back on the rota. The 3rd 8th and 11th were changed. I would say your late 50s for the change would not be too far off.
The tee for the old 8th 183 yard hole was too the left of the 7th green up on some high ground and so would have been out of the view of the painting scene. The plan of Sandwich that I took from my 1949 Open programme shows more bunkers on the 7th than present, a few more on the left, though these may be concealed in the painting.