I suspect that the address at 135 was a 'mothering' address. There are rumours that her mother got pregnant by the son of the family she was in service with, and talk of him not surviving the War.
It would be an idea to list all surnames at 135 Paisley Road West from the VRs in 1915, have a look at maternal matches, their trees, shared matches & their trees to see if any of those surnames from the VRs features a lot?
Surnames on the 1915 VRs...
Brown - Clark - MacKenzie - MacLeod - MacTeague - Montgomerie - Neave
The Proprietor was John Mair
It may also be worth looking closer to home to find out whether the family who fostered Agnes were related in any way?
What occupation did the foster father have & how far from Govan where Agnes was born did the foster parents live?
It's worth keeping in mind, although a lot of work has been done so far to trace Agnes' roots, it's all speculation as there's no solid proof of anything beyond the BC unfortunately.
That's where using DNA matching can help.
Annie