Tri-leg Sugar/Salt [S] [T] [S] Added mark (A 1)

[Traintime]

Not among British plate listings, but there is one with STS letters in shields. First three in octagons. Last encircled appears to be "A 1"...only right leg of A is visible. TIA

[Traintime]

More:

[dognose]

Could it be that due to the curvature of the base that one, maybe two, letters are missing from the start of the mark?

Also the last letter 'S' may not be part of the name, but perhaps an indictor of 'Sheffield'?

Trev.