The more expensive rails have ball bearing slides so once mounted, you can slide the server out, do some work, slide it back in. No need to re-rack. Static rails don’t move. You either put the server in or you take the server out.
I have 8 of those from that seller running as four mirrors in my proxmox host. Very happy with them and in my very limited knowledge, they do seem to be one of the few SAS enterprise SSDs at decent prices.
I wanted to pop back in here and thank @Almighty as well. I received the Supermicro 3.5 to 2.5 toolless drive adapters and they look completely unused. On top of that, I was expecting adapters I’d put in my current 3.5 drive caddies, but these were brand new 3.5 drive caddies with the toolless...
The sticker on the chassis seems to indicate the motherboard is an X11SSL:
It could be this server: