Bolivia: T-Stamp In Negative ; Use?

Hello,
I bought this cover because of the bisected 10 cantavos stamp.
On the back there is a La Paz cancel dated oct 13, 1919, but nothing else.
So why and where came the T stamp on the cover?
Hoping for elucidation
Heinrich

The "T" is a typical indicator for postage due - Taxe Percue.

I believe this to be a postage due marking Heinz

Chimo

Bujutsu

thank you, battlestamps and Bujutsu, that is what I supposed, too.
But: a) this would be the 1st time I see it in negative and b) in 1919 things were not so lax as they are nowadays ... somebody in the USPS would have noted the amount due, I suppose, and it would have been collected from the recipient. Of all this no trace.
Or am I totally wrong?

Not all postage due mail had markings on the individual letter to show the fee paid at the time of delivery. I would expect that to be even more true for an import company the size of Arkell & Douglas, which probably had stacks of letters from various parts of the world that could have been delivered with inadequate postage and was likely paid together rather than individually by each piece of mail.