Richard,
Don't neglect Durst masks. When I got my 504 I found that the masks I had from my Durst Laborator 54 fitted. The external dimensions and method of fitting were exactly the same. Possibly those from a Durst 138 may fit.
For example see
http://cgi.ebay.co.uk/Durst-Laborato...#ht_500wt_1156, especially the note at the bottom re using them on a DeVere.
Bill