This is one of three kits sold in 2002 that aren't new sets but is a collection of previously sold Harry Potter LEGO® sets. I am guessing that Lego did this so that people who wanted multiple sets could get them in one purchase, but I don't think they were all that popular because they only did this for a few years.
I have never seen this kit listed on either Amazon or eBay but I've included links to both sources just in case you get lucky. However, it will be much easier for you to buy the sets that were included in this kit separately, which is why I've included links to each set in the description.
Set K4701 - Chamber of Secrets Kit
Year Released: 2002
Set Number: K4701-1
Name of Set: Chamber of Secrets Kit
Book Set is Based On: Harry Potter and the Chamber of Secrets
Sets in Kit:
4705 - Snape's Class
4709 - Hogwarts Castle
4730 - Chamber of Secrets
This LEGO® set contains small pieces that could be a choke hazard and is not meant for children age three and younger. LEGO® is a trademark of the LEGO Group of companies which does not sponsor, authorize or endorse this site.
