Horizontal Steam Engine Problem - how does it work?

I've been asked to sort out a model of a lancashire mill engine and I can't for the life of me understand how it was meant to work. The engine is a single cylinder steam engine with the steam chest on top of the cylinder. Instead of the usual single slide valve arrangement (a la Stuart Victoria), it has two rods driven from individual eccentrics, but the eccentrics throw is aligned ie there is no angular difference in the throw of the eccentric. Through an arrangement of levers two rods for operating the valve gear emerge one vertically above the other. At this point the timing of the action has moved from being in phase to being in anti-phase. The lower rod drives what appears to be a conventional slide valve covering three ports, though its difficult to be absolutely sure without stripping this down (I can't get at the underside of the valve). The upper pushrod is connected to nothing in the steam chest, whatever valve it was supposed to drive is long gone. There is too many hours work in the model for all that assembly to be a dummy (assumption).
I've drawn a complete blank as to how this valve gear was intended to work or what the missing valve looked like - hours of fruitless Googling has revealed nothing like this.
So I've tried to sketch the valve chest side on
www.btinternet.com/~steve.withnell/lathe/chest.jpg
And provided this photo of the valve gear arrangement:
www.btinternet.com/~steve.withnell/lathe/pushrd.jpg
Any ideas gratefully received -
TIA
Steve

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Since its a mill engine it wont need reverse, and thrifty(tight) mill owners wouldnt put extra into a machine for nowt. Could it be some sort of inlet regulator?something that would allow you to 'notch up' the engine, a bit like the slotted link in a stevensons gear?
Dave

Steve,
If one eccentric is for the slide valve in the steam chest, it may be the other one is/was for a water pump. But that's only a guess.
Mike
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It looks as though it is intended to have an expansion valve. I do not have time to explain right now.

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On second thoughts, look at this:
http://oldenginehouse.users.btopenworld.com/expslv.htm

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Thanks Charles, I'm beginning to wonder if an expansion valve was intended but not implemented fully. To scale the picture, the steam chest screws are 14BA - ish (not yet measured them up). So producing even a manual expansion valve that worked would have been a challenge at the time this was built.
My thought was that the expansion valve should move in phase with the slide valve?
Steve

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I wondered if it was a patent prototype to show a novel linkage to allow an overhead slide valve, with the internal valve mechanism being taken as read. Perhaps overhead slide valves have an advantage as regards handling condensation or lubrication.
Just a thought..Steve