You're quite right. The 18 mo's must be in combination with the "quarters" as shown by the Mexican equivalent.
We will be forced to accept a proven corrupt, lying, cheat government's word for their contributions and you can wager it all upon Mexico's not hesitating to "enhance" their records if it comes down to American taxpayers ending up supporting Mexican retirees and all their other "qualified" recipients.
Not going to happen but raise the alarm anyway.
If you have a chance read this:
http://www.gao.gov/new.items/d03993.pdf
The Government Accountability Office takes Jo Anne Barnhart to task for only going to Mexico for 2 days and signing the agreement. She is the Bush appointee for the SS administration who signed the agreement.
Read the first couple of paragraphs, then scroll down to 13 of 35 then the chart on page 16 of 35 which tell of the under and over estimates of the previous countries. This article is an eye opener itself.
This is a non partisan article as both the GAO and SS debate the issue on this url.