Yes it looks to be Dative type, it looks like it says, "Testamantium Dativum L....". It is definitely the 1639 document. I cannot decipher much of what is written over its two pages.
Is it all in Latin, then? That bit is definitely Latin.
I think the 1580 Date of Birth for Alexander Laird was from a record on ancestry .com, if I recall correctly it was listed as "Alexr. Laird". I'll have a look.
Hmmm. Sounds as if someone was guessing that he was about 50 when he died in 1639.
It occurred to me that even though Alexander Laird is described as 'in' Cassiltoun rather that 'of' Cassiltoun, there might be something in the index to the Angus Registers of Sasines, of which I happen to have a copy. There are a couple of Lairds, but they don't shed any light:
Laird, James, in Meigle, formerly in Potento (S.3) IX.296 (bis); X.98; his spouse, See Gray, Jean.
Laird, James, in Newmilne, (S.3) X.175.
Gray, Jean, spouse of James Laird in Potento, (S.3) IX.296
(S.3) is the 3rd Series. Vol IX covers 23 March 1692 to 20 August 1698 and Vol X covers 23 August 1692 to 12 December 1700. There are three Sasines mentioning James Laird in Meigle, one of which also mentions his wife Jean Gray, and one Sasine mentioning James Laird in Newmilne. No mentions of Alexander Laird or Castleton of Eassie, and they are half a century too late to be relevant. I don't know which Newmilne is referred to, or where Potento was.
Meigle is in Perthshire. There could be references in the index to the Perthshire sasines, but I don't have a copy of that.