That's a good point, that the clear canopy doesn't fit the grey X-Wing. I have never tried to swap them over. The idea of a different factory rings true, too as Meccano also apparently produced the X-Wing later on and was simply assembled at Palitoy http://forum.rebelscum.com/t1003493-3/
I have done some more research and I believe I have come up with a definitive answer on the release of the Palitoy grey X-Wing. The grey Battle Damaged X-Wing was released in America in 1982 but was unlikely to have appeared in the U.K at that time because the tooling moulds needed to be sent from Kenner. Instead, Palitoy simply included the stickers with the white X-Wing and airbrushed the Kenner box photos. The 1982 Palitoy trade catalogue has pictures of the white X-Wing. Catalogue photographs from Argos, etc also show the white X-Wing.http://www.brightontoymuseum.co.uk/info ... at1982_p05
Palitoy, or more probably Meccano received the Kenner moulds and prepared its own version of the grey Battle Damaged X-Wing ready for 1983. The 1983 Palitoy trade catalogue pictures it for the first time. The reason why the Palitoy grey X-Wing was not derived from the Palitoy moulds was probably that Meccano ordered the Kenner ones.
The grey X-Wing was first issued in the ROTJ Bi-logo box, which is fitting for a Palitoy/Meccano product. There were stocks of the white X-Wing left over, so it was commonly sold in that packaging too. Catalogue photos from Argos, etc in 1983 still show the white X-Wing. In 1984, the catalogues show the grey X-Wing.viewtopic.php?start=15&t=12059
Perhaps there was left over packaging from The Empire Strikes Back in early 1983 which was issued with the grey X-Wing but essentially the grey X-Wing was a Return of the Jedi product.