Toilet syphon

[MANDATE]

Examined a toilet syphon today which would not flush and found some form of housing about 30 mm from the vertically moving piston rod.
The housing was slotted front to back and within there was a hole from which water spurted when attempting to flush.
Temporary sealing the housing with insulation tape enabled flushing to take place and proved the piston/diaphram to be effective.
Does anyone know the intended purpose of this housing? All the syphons I've seen did not have this and as yet I have not come up with a logical explanation.
If the hole is blocked up, then there was no point in having it in the first place.
If the hole is left open it prevents flushing, although it may not if it was a smaller hole.
It could be part of a spring loaded valve ( with bits gone missing ), but why would that be needed.
Any ideas?

 

[breezer]

picture?

as yet no idea, but i suppose when you find out you will be well flushed

 

[PTH]

Could it be a form of internal overflow? I fitted a new syphon recently that had a couple of rubber bungs near the top bend with instructions to remove for internal overflow. 'Great' I thought - I can remove the pesky overflow pipework that gets in the way of the stopcock. However the flush would not work with the bungs removed so I had to replace them and the original overflow pipework (rummaging around in the dustbin at midnight got the neighbours chatting I expect).

 

[MANDATE]

No it can't be overflow related! Having given it more thought I'm thinking it is some form of duo flush device, perhaps it was a spring loaded ball valve where the piston opens the valve when it reaches the top of its stroke and if held there only gives a shortened flush (air could enter at higher level). If lever is released it gives a full flush.
Even if I'm wrong it sounds good and I can't think of any other purpose.