I just received the Autumn 2009 edition of Cross Post, the semi-annual publication of the Friends of the British Postal Museum & Archive. In it, Don Staddon writes about silver Machins dating from 1994.
The BPMA received a number of contributions from Royal Mail's stamp section after it moved offices (never too late to do a little house cleaning!). Included in the hoard is a booklet of 12 x 19p Machins printed in metallic silver. The cover is similar to the ones used today, except that it is
also silver in color.
Staddon says that the stamps were printed by Harrison and Sons and then mounted in covers presumably produced by Walsall. I wonder if we'll ever see issued stamps in this color.
Incidentally, this is a good opportunity for me to plug the Friends. It's an organization that supports the BPMA. The BPMA has grown considerably since its formation a few years ago, and it is now preparing to open a true postal museum in 2013. The BPMA has an excellent web site here and a blog here.
Information about the new postal museum is here. There are two online exhibits of interest to Machin collectors here and here.
If you'd like more information about joining the Friends, see here. I strongly recommend it.