D&L's Kris-Por mi Lei Y Por mi Rei

[fernando]

I don't think this inscription is written in either old Spanish or old Portuguese.
The use of the i in these cases is a 'modernism'; only 'recently' it replaced the y in Portuguese; in Spanish (Castillian) the y prevails.
The word por may be either Spanish and Portuguese. The word mi will be only Spanish.
On the other hand it doesn't make sense the use of the y in the middle of the phrase and the use of the i in Lei and Rei; it is either one or the other.
It could be that the guy who wrote this inscription intended to write in Spanish; but he was familiar to 'modern' Portuguese.
... hope i made myself understood .