Most 12 lead ECG's are random captures of cardiac activity. For STEMI cases, the 12 lead will do but for arrhythmia recognition longer strips are needed.
Image 2 - covered 2:1
Image 3 - 2:1
The strips in images 1 and 2 are intentionally covered to simulate what you see in a 2:1 AV block. Image 3 is the 10 sec full disclosure strip. All manifest 2:1 AV conduction. Most would right away jump into the conclusion that this is second degree AV block type II (Mobitz II). Be refrained in labeling such strips as such until you see a longer strip.
Image 4 - Image 1 uncovered
Image 1 is a Mobitz I
Image 5 - Image 2 uncovered
Image 2 is also a Mobitz I
Image 6 - Image 3 transition strip
Image 3 is a Mobitz II
You might think that Image 2 which shows a right bundle branch block (RBBB)configuration is a Mobitz II but transition strips indicates this is Mobitz I. The RBBB in this case is fixed.
A 2:1 AV block is unique form or subtype of second degree AV block. This is not Mobitz II. A 2:1 AV block can either be due to Mobitz I or Mobitz II. YOU NEED A LONGER STRIP TO NAIL THE DIAGNOSIS.