That is a splendid find. Top marks to you for noticing it before the bulldozers blade swept it away.
I've come across a few similar benchmarks cut into what in this case looks like a boundary or property division marker stone.
The OS Benchmark list for this grid square TQ 3678 gives this as the description and level for this mark.
TQ 3619 7878 CUT MARK STO SW SIDE YEOMAN ST 70.0M SE CHILTON GR 0.4M SE WALL JUNC 1.782 3 'N' 1991 1991 0.100
The date of 1991 was the date of the last Ordnance Survey re-levelling.
The stone was there first and the OS Surveyor was looking for a site to cut a new benchmark and choose this position.
I'm also fascinated by the inscription, which as you say has 2 dates 1859 and 1887 along with the letters S with PD below and S with MB below.
My guess is that this stone was emplaced by the developers of these premises to exactly mark the boundary division of the two adjacent properties.
I've checked the old OS plans of the area which show this Benchmark with the height above sea level as 6.08ft, but the stone isn't annotated as a boundary stone or anything .
In regards to your question about conserving this mark, many similar benchmarks have been lost as development progresses. The benchmark, once the stone has been disturbed is of no value as a benchmark, but the historical relevance of the marker stone may well have.
Title Deeds of this area might reveal the significance of the inscribed letter and dates.
I for one would hope that this boundary / marker stone could be preserved and perhaps rebuilt into a wall in the near vicinity, along with a record and photograph of where the stone was originally located.
So there are a few pointers or lines for further enquiries.
Please come back and it would be good to hear whether this stone could be preserved for the future.