That's quite interesting...

In their downloadable PDF catalogue ( Here ), there's a spreadsheet that tells you which lamp to order when you have a lamp from other manufacturers. At the page 33, in the mercury vapour section, to replace Sylvania's /N lamp they recommend to order ... a /WDX lamp! The HR100WDX38, HR175WDX39 and HR400WDX33 are listed in the cross-reference, but not anywhere else.

The catalogue has been updated in 5/2010, so I don't think it could be an error. I hope it itsn't! I also contacted GE about that point, can't wait to have the answer!