It's not exactly answering your question, but there is a brand new never used Nacra 5.5 square topped main in stock at the dinghy store, and I know that the owner doesn't know what to do with it as nobody is sailing a 5.5 anymore around here. I don't recall the specific info about the sail, all I remember is that it doesn't fit on my 5.2 'cause I measured it a while back. But it looks like a boomless squaretop mainsail, and it is definitely for a Nacra.

It's in the UK, but hey, you may be able to ask a good price as it has been sitting here for a while... It's not listed on the site, but you should contact them: http://www.dinghystore.co.uk/