The Gemoro Rosh Hashana 11a says:
Chodesh זיו is the month of אייר. The reason it is called Ziv , is because the Avos were born in Nissan.
By the time the month of Ziv (Iyar) arrived all the Avos were already born . (see Rashi)
The Rema (E.H. 126:6 ) writes:
“Since there is a Machlokehs how to spell אייר (if 1 or 2 Yuds) we try to refrain from writing Gittin, the entire month .”
Reb Akiva Eiger quotes the Yalkut with the ראשי תיבות of אברהם, יצחק ,יעקב ,רחל
From the ילקוט we see that the spelling of אייר is with 2 Yuds.
The אלשיך in Parshas ויחי writes " the Merkava requires to
have all three אבות both Yitzchok & Yakov (two yuds)
Hence, this explains why the ראשי תיבות of of אייר
is the סוד המרכבה