There is not and never has been a plack that stated, "The Secret of the Universe is 7129/6105195".  This phrase actually started in jest amongst the members of the old Coral Castle forum. 

As you can see below the numbers are etched into the doorjamb of Ed's living quarters.  It was often the practice for immigrants of the time to display their naturalization numbers in this fashion, especially around WWII when the patriotism of immigrants from certain regions was in question, to put it lightly.

Transcribed versions of Leedskalnin's writings are available here.

Perpetual Motion Holder Scematics and Leedskalnin's Original Writiings
Dave Nelson's Coral Castle Commentary
What Ed Wanted You To See
Castle Photos
Perpetual Motion Holder
Castle Photos
Ed's Pics
Tips For The Rocket People
Dave Nelson's Commentary on Leedskalnin
Ed's Generator
Leedskalnn's Writings
Magnetic Universe