I know it's an old thread but I think I've an answer...
I've been collecting British webbing since the 1970s and I firmly believed the same until lately. That said, I've never understood why mess tins were always stamped yet not '44 waterbottles. Not a single one.
I was a Paratrooper reservist in the 1980s and 1990s and I worked in the company QM stores as often as I could. We had both types of '44 w/b and both types of '44 mug. A mix of '44 and '58 waterbottles (as desired, if availability allowed) were still issued right up to 1990, when the S10 respirator cap needed to be fitted to the webbing carried w/b (but not the spare carried in the bergen).
By that time, of the w/bs, what remained were knackered: dented to hell, leaky or permanently jammed into mugs - King Arthur may have drawn the Sword In The Stone but he would have struggled drawing the "Waterbottle In The Mug"...
On leaving, I handed in a '58 mug instead of my trusty '44 - which I still have. It's unmarked like many of the mugs that were in stores. I thought that that was 'cause it was one of the many items that 5 Airborne Brigade had made by special contract. I know that 5 Ab Bde had '44 waterbottle pouches made (with 1980s ordnance markings) because we were waiting on their issue for many months.
When I was issued a '95 mug (aka "Crusader cup") in the late 1990s, I consigned my '44 mug to my van emergency kit box and forgot about it. Digging it out recently, I took it to be an imitation and not the one I was issued in the 1980s. Checking photographs, it turns out to be the one I was issued and identical to the unmarked one on Karkeeweb.
I've also a marked '44 mug (MMS dated 1961). Dates seen on mugs so far are 1945, 1961, 1964 and 1965. Perhaps these mugs were made only by one manufacturer (MMS), the unmarked ones by others?
I recently acquired a Danish Army M59 waterbottle for bushcraft - unlike the British waterbottles, it's ordnance stamped on the base. It looks identical to a '44 Mk1 but for it's plastic cap. Doesn't fit properly into either of my '44 mugs, though, nor my '95 mug. A pity.