Today, when I was learning how to assemble/disassemble tefillin with my sofrus tutor, we opened an old pair of my tefillin and found that the batim were lined with what appears to be posterboard or plastic of some sort, glued into the bayis. The only part of the batim that was exposed leather/klaf was the flap of the titura, and, on the shel rosh, the klaf separating the compartments (the tefillin are peshutim mehudarim.) My tutor had never seen anything like it before.
Has anyone seen this sort of thing before? Thoughts on the kashrus of the batim? My tutor was of the opinion that this would constitute a chatzizah, but allowed that there might be a kulah of which he was not aware.
Pictures below, for those who are interested. I apologize for the quality, as I was shooting the pictures with my phone's camera, which is not quite designed for this sort of work.