I'm sharing here for interested people some research performed by Countess Aryanhwy merch Catmael on whether rulers in their own documents referred to themselves as "the third," "the fourth," etc.
Some months back there was a discussion about whether regnal numbers were used _by the ruler him/herself_, or only in 2nd-hand references by, e.g., historians. I'd said I'd found evidence of the former. Today I found another example. In the Hamburgisches Urkundbuch, there's two documents from 988 from Otto III which are dated as follows:
"Data XVII kalendas Aprilis. Anno dominicae incarnationis DCCCCLXXXVIII.
Indictione I, anno autem tertii Ottonis regnantis quinto"
"Data XV. kalendas Aprilis, anno dominicae incarnationis DCCCCLXXXVIII.
Indicatione I. Anno autem tertii Ottonis regnantis V".
The last clause in each indicates that 988 is the 5th year of the reign
of the third Otto.