Hi Don, this may help if the construction is similar to K&D.
First give the unit a good soaking with this PlusGas http://www.ebay.co.uk/itm/1-X-Plus-Gas-Tin-Spout-250ml-Bolt-Screw-Assembly-Lubricant-Clean-Protect-/391096277473?hash=item5b0f2799e1or equivalent. This is the best penetrating fluid I have come across.
He is a drawing of a K&D unit which I "THINK" is similar to yours.
If you look at the right hand cross sectional drawing, remove the screw "23" first. Now the remove the locking wheel at the rear "21".
Once you have removed these parts, Use a brass or aluminium drift to knock out the assembly "Fig5" shown at the lower left. This will also remove the circular die plate at the same time.
Give them all a good clean then lightly lubricate and reassemble.
I hope this works for you.