Only a 20ft container not 40ft. I was always under the impression a 20ft was roughly 2.2-2.4t empty. Which must be half right as lifted one with a telehandler on chains once which could only lift 2.5t max and less with boom out but it lifted it. Just…. Ha!
A place I worked at once had some of them, as
@JD450A says international tractor based, mast bearings were pricey we could only find OE at the time.
That photo looks a later one so could be a Perkins and shuttle box (earlier were shuttle but manual with a clutch).
Your right with the container weight, from memory these machines are rated at 2400/2600kgs at 600mm centers, you`ll be at 1250mm centers with a box

. You can get higher capacity versions but mainly new models.
They won't spin round like a normal FLT, a 4ws telehandler of the same capacity will be more maneuverable, but if you`ve got the space/ground not suitable for a normal FLT they are a good valve option.