It is usually a Minilite 15x5.5 and as Calum says, with a specifically drilled and machined centre. Some have a Team Dynamics 15x6 wheel, that has a poor clearance to the rear drive sprocket and belt. It doesn't fit well but this didn't seem to bother Morgan much. I don't know what the offset is. You could try asking Minilite or Team Dynamics but you may find they have the usual Morgan "gagging order" if you mention it is for a M3W. I will be interested to hear how you get on and if they are available other than via MMC.
Just had a look on the Minilite website and they say they can supply different offsets and PCD's. If you can measure one up it should be possible, although the hub/axle clearance fitting is non standard.
http://minilite.co.uk/technical.htm. 
