MAF Sensor Size - Internal Or External?

Been researching MAF sizes a little and wondering if the sizes I've read about (2.5", 2.75" & 3") whether those measurements are internal bore diameter or external pipe size?

Mine measures 70mm or 2.75" can someone confirm that I've measured it at the right place?

Also the part number is 06A 906 461 LX.

Think I'm just confusing myself [emoji1]

Thanks for any help.