Confusing specs for Latitude 11 5175

I have a latitude 11 5175 but wish to increase the size of the SSD to 1TB. Several reviewers state that the maximum drive size is 512GB. How do they know this or is this only because that is all Dell could fit due to availability at design time ?

Secondly the only drive I can get that is 1TB has these specs -

SSD M.2 1TB PCIe NVMe. It happens to be an Intel.

Dells spec from a brochure claim this -

SSD M.2 512G PCIe NVMe (March 2016)

What on earth does this mean ? Although my unit does have a 128GB I haven't checked yet whether it is just an M.2 SATA. So can I make an assumption that as long as my unit was made after March 2016 that I can fit the PCIe 1TB unit ???

Note I haven't checked yet)