Found front images of George Washington and U.S. Grant cards. Also, #8 FDR, #9 Truman.
The numbering looks to be in chronological order. Here's a logical guess on numbering:
1 - George Washington
2 -
3 - Abraham Lincoln
4 - Ulysses Grant
5 - Grover Cleveland
6 -
7 - Woodrow Wilson
8 - Franklin Roosevelt
9 - Harry Truman
10 - Dwight Eisenhower
11 - John Kennedy
12 - Ronald Reagan
For card #6, the Presidents between Cleveland and Wilson are McKinley, Teddy Roosevelt, and Taft. The only Presidents after Reagan are Bush and Clinton. So the checklist probably ended at #12.
IMO, I'd go with the above checklist and put "TBD" in #2 and #6.
Someday my cards may double in value and then be worth half of what I paid for them.