Anime-Beta TNG

As I've been removing tape from all my pieces, I've been doing a complete reorganization of my collection and I've found two cel books of unknown heritage. One looks very similar to an Itoya (what all my others are) except it's page inserts are light grey instead of black. The other is definitely not an Itoya - it lacks page inserts at all and has a weird hole (obviously cut there when it was made, not damage) in one corner of the back cover, under the clear plastic for a spine insert. Both are an odd 12inx16.5in size that I don't think Itoya carries.

My question is: Could these unknown cel books present a danger to the pieces stored in them? I only hesitate to get rid of them because their weird size is *perfect* for a certain number of my sketches. I've had them for years (5-6 years or more) with no apparent damage to anything. Thoughts? Swap them out, or let them stay?

There are a lot of different companies that make books for storing artwork out there. Itoya just happens to be the popular one.
If you have stored items in them for 5-6 years with no damage I would expect they should be fine to keep. The only question might be if the paper is acid free. Maybe as part of your reorganization you might consider changing the paper to a known acid free paper.