I had a little rummage around the web to find some photos of the 64xxs. Here's a selection of the nicer ones - thought I'd share the links here in case anyone is interested.
Both Russell's GWR locomotives and the Wikipedia site for the 64xx notes that one way to tell the difference between a 64xx and a 74xx was the join between the bunker and cab, with the 64xx class having a curved join and the 74xx having a straight join. This seems to be wrong. As quoted by Nick in the main Bachmann announcement thread, the RCTS volume states that no. 6430 was the first 64xx to have a straight cab/bunker join. This seems to fit with the photos found so far, except no. 6412 below, but it may have been modified during an overhaul?
This has certainly whetted my appettite - not to mention modifying one into a 74xx!
Selected photos on the web:
No. 6403 at Snow Hill, 1937, cab/bunker join not clear, busy little engine!
http://www.warwickshirerailways.com/gwr/gwrbsh1793.htm
No. 6407 (the one Bachmann are releasing in GWR livery), curved cab/bunker join, BR early crest:
http://www.flickr.com/photos/phil-seaford/8208071267/
No. 6412, rusting away at Swindon, 1966 (?), straight cab/bunker join (?!):
http://www.flickr.com/photos/60790501@N04/5809232761/
No. 6422 at Snow Hill, 1936, curved cab/bunker join, GWR Shirtbutton
http://www.warwickshirerailways.com/gwr/gwrbsh1762.htm
No. 6430, preserved, straight cab/bunker join, w. auto coach:
http://www.flickr.com/photos/rabbipx/6306401575/
No. 6435, 1960, BR early crest, - if you don't like this scene you're not human :-)
http://www.flickr.com/photos/taffytank/8428258962/
No. 6435, preserved straight cab/bunker join, lovely action shot:
http://www.flickr.com/photos/taffytank/5773103086/
No. 6437, Gloucester, 1963, straight cab/bunker join late crest lined out:
http://www.flickr.com/photos/16749798@N08/4166048684/in/set-72157617567479618
No. 6438, straight cab/bunker join, GWR shirtbutton:
http://www.probertencyclopaedia.com/photolib/transport/GWR%2064xx%20Class%20Locomotive%201955.htm
Drawings and overviews on the web:
64xx and 74 xx drawings:
http://www.champwilde.f9.co.uk/gwdrawings/locoindex.html
Jim Champ's "beginner's guide to GWR pannier tanks":
http://www.gwr.org.uk/nopanniers.html
Edited to add/modify links.