Help Identifying Fitting on Hot Water Cylinder

[RCr]

My solar hot water cylinder does not have the usual drain cock on the cold feed. However it has this 15mm compression fitting at the bottom with the small hex fitting on the underside. There is a blanking disc in the compression fitting.

Is this a drain off fitting for the cylinder or something else? I have not seen this set up before, and certainly not the small hex fitting on the compression fitting.

 

[CBW]

There are several types on the market, looks like a drain off to me.

 

[Madrab]

Is it a dual coil cylinder? I.E. does it have a normal CH coil and a solar coil and are both being used?

Wouldn't normally expect a cylinder to have 2 CW entry points or a separate drain tapping, though in this game, never say never.

 

[RCr]

Yes its a dual coil and both coils are connected up. Like you say I've not seen a separate drain on a cylinder before, nor that particular fitting but it does seem the most likely thing.