What is this component connecting four CH pipes?

[senfre_]

Trying to understand my current installation before I make some minor changes. There is a component, sort of like a plumbing junction box, which connects four pipes. Could anyone tell me what this is?
In the labelled picture attached:
1: Flow from boiler
2: Onto pump, mid-position valve & rest of system
3: Not sure, probably CH expansion (goes into loft)
4: Not sure, probably CH feed (goes into loft)
If it helps, I believe the system was installed in the mid-90s

 

[blup]

Myson air jack (?), takes air out the system supposedly, mine never did and the plumber took it when the water tank was moved

 

[pcaouolte]

Myson Aerjec II air separator.

 

[senfre_]

Excellent, looks bang on. Thanks chaps

 

[stem]

 

[gas112]

They have a habit of blocking up with magnetite so if some rads struggling it will prob be a restriction in there and needs cut out and altered

 

[senfre_]

View attachment 333186

Either mine's plumbed wrong, that diagram is wrong, or I've got a different model, because the cold feed and flow are the other way around on mine..

 

[pcaouolte]

the cold feed and flow are the other way around on mine..

That won't matter.