Master Page Gallery is a page library that holds all master pages for your site.
- Get the Master Page Gallery.
- Compare all items with the content type Id of “Master Page” content type id
- 0x0101050038D0E9CC383A4C4B8647D37D80458A8A is content type id for master page.
- All matching items are the master page.
aanuwizard.com 
not so sure on this issue can you please check it again? before posting into your blog!!