S400f updates?

[Bezzer]

I've got an S400f Classic, believe it's a 2009 going by the cylinder date, the upper receiver/bolt housing part is secured by the 4x M3 machine screws from the top and the 2x M4 anti tamper covered machine screws from underneath, The AT plugs went a while ago
Was at my local HFT shoot yesterday and was partnered up with a bloke with an S400F carbine, he was asking about getting to the hammer rail to give it a bit of a clean up, I started to point out what to do then realised his upper receiver was totally plain, there were no allen heads nor the 2 open holes from the underside screws to be seen. I never asked how old it was.
I take it that at some point the 400 was updated and everything is secured from underneath now I've got a bit of a project in my head that I will hopefully have a go at later in the year involving a 400 and I might have to use another upper receiver part, I take it the two types are not interchangeable and were there any other updates/alterations to the gun ? I'm just after knowing so I can bear it in mind when I start getting the bits together I need,

[retroruss]

i have 3 400s all different
1st is a 2004 with all breech block bolts m3 all 6 are accessible from the top
2nd 2009 4 bolts from the top and 2 m4's from underneath like yours
3rd s400f 2016 all bolts are accessible from underneath one is an m4

i prefer the 04 model / design myself it shoots the best as well

[Bezzer]

Thanks for that, I'll just have to bear the differences in mind if I do need another one, they obviously won't interchange without a fair bit of alteration.