That Pen L1 is rather interesting. The specs listed are a little weird though - It looks to use a specialty internal USB charged 10900, but the text also says it runs on 10440 and 10400, depending on where you look.
I suspect that they copy and pasted from the C1 information and didn’t proof read very thoroughly. The published output specs are the same for the C1 and L1, and they probably shouldn’t be the same.
It does look like it uses a really special-looking 10900 lithium ion battery, which has a built-in charging port (otherwise, no current charger can charge 90mm length batteries (maybe a magnetic charger with a low-charge current will work)
I wonder if the 10900 is about twice the capacity of 10440, since it looks to be about twice as long?
Or maybe can we substitute 2x AAA NiMh/Alkaline batteries (but voltage may be too low?)
Ok, I supposed if the 10900 battery goes bad, then another option could be to use a 10440 battery, and then use a dummy AAA battery to connect with it then…