Big red stain on f103r and retouching of text

[Botis]

I wondered if it was caused by the Marci letter, that may have been preserved between the pages, in which case this could a stain from the wax seal.
An other suggestion I heard was that it could be wax of a candle that dripped off during an evening reading session.
There are a few more in the MS but not many.

Colored candles were very rare before the 1870s , so if it were colored candle wax it would have to be late 19th or 20th century.