First, welcome to the Forum.
You will want to check carefully with your perforation gauge to see if it is
the flat plate perf 11 (Sc. 562)
or the rotary press perf 10 (Sc. 591)
or the later rotary press perf 11 x 10 1/2 (Sc.#642.)
Further details at:
http://www.stampsmarter.com/1847usa...ntifier.html In this particular case I suggest using the keyword search feature. look for Monroe. Not all that many stamps and varieties featuring that particular president.
If you do not have a perforation gauge, as you can see - you will need to get one, or find one online that you can print out.
Hope this helps.
- Jonathan