Ferryhill's Mystery Masonry; Enigmatic "S"

A little while ago, we posted about "unknown stones", and in the post, we included this picture of an "S" marked stone in Ferryhill.

At the acute angle meeting of Ferryhill Road and Ferryhill Place

We had assumed that, like so many marked stones we see around Aberdeen, this was a boundary stone, marking someone's commercial or political property. These stones are very often shown on the large scale Ordnance Survey maps, but there's no sign of this one on the 1869 25 inch-to-one-mile map. So, it's later than that...

The map shows a couple of 'stones' on the northernmost curve of Ferryhill Road. That'd be close to where the defunct public toilet is situated. We hear that the toilet is still in occasional use; No 17 bus drivers having a key! Brilliant!

Stop the bus!

Walking down (northward) Ferrhill Road from Ferryhill Place towards Crown Street, we see these:

This one's got an OS Cut-Mark and Rivet Bench Mark,
down at the bottom.

They're all the way down the road, each about 50m from the next. A neatly incised "S", surrounded by a perfect rectangular border, on a piece of flat-faced granite - ashlar at the top (southern) end of the road, rubble at the bottom (north). The bottom (north) end of the road is characterised by a large retaining wall which encloses the steep-sloped common green of Archie 'Pech' Simpson's Marine Terrace.

By the time we reached the bottom, our eyes were well-opened, and we also noticed this wall-bound artifact:

"Presented by
Andrew Lewis Lord Provost
1927"

So regularly spaced, so unlike the other boundary stones; these "S"-marked stones are perplexing. Often, boundary stones are marked with the initials of the landowner, like this one at on the top of the Broad Hill:

Marked "R" to the north, "ND" to the south. Grooved on top.

So these stones in Ferryhill aren't the usual Aberdeen boundary stones, they're quite different. We've got a few theories:

• We've often noticed the words "sundry proprietors" marked on older maps of our town, and we wondered whether the single "S" might refer to that. 
• My old dad thought that the "S" might stand for "Simpson": that's Archibald Simpson, who built Marine Terrace, up above the stones.
• Someone suggested an infrastructure connection, referring to Soil- or Sewer-pipes.
• Similarly infrastructure related - Aberdeen's first source of electricity - the "Corporation Electricity Works" is at the bottom of Ferry-Hill, and so at the bottom of Ferryhill Road. Might the "S" be something to do with that? Substations?
• Aberdeen's first electrified tram ran up Ferryhill Road to the south and Crown Street to the north, it's source of power being the "Corporation Electricity Works" at the bottom (south) end of Crown Street. Perhaps the "S" is related to the trams?
• Ferryhill Road connects with the bottom of Crown Street, and so is close to the Masonic Temple. Amongst many things, the Masons are/were know for "masons marks". Is it possible that the "S" is a latter-day and more literate version of these ancient esoteric signs?

I grew up and went to school in this specific area, and I played on Ferryhill Road and the Marine Terrace green countless times. Since my childhood I've walked up and down Ferryhill Road regularly. It's amazing to me that I'd never noticed these plain-as-your-face marked stones before the day that we walked down Ferryhill Road looking for anything out of the ordinary. 

Neither our local council's Sites and Monuments pages, nor RCHAMS "Scotland's Places" pages mention these "S" marked stones on Ferryhill Place. So, if anyone knows for sure what they are, or has any other theories, we'd be delighted to hear about it.