The system boiler appears to be used on sealed systems only!
Can someone explain why please?

Manufacturers have different ideas of what system means.

However, almost all of them agree that it will include a pressure vessel,
pressure gauge, pressure relief valve etc. for sealed operation. Most (but
not all) also include the pump and filling loop.

Is it likely that the system will be "sealable". What would make it
unsuitable for sealing?

If the pipework starts weeping at the joints, then it wasn't suitable.
However, this is just bringing forward the inevitable. You might as well
have the leaks fixed now when the plumber is round, rather than letting them
develop naturally and have them express themselves on the first day of your
foreign holiday next year.

1970s plumbing would definitely have been designed for the pressure. Only
faulty installation or serious corrosion will have let to them no longer
being suited. Do note that during the copper shortage of the 1970s, pipes
were made thinner and may not have their full longevity.

Christian.