The seals there, exactly where it should be, sealing the sender unit to the gearbox.Your seal appears to have fallen off….
New* tacho senders/heads use encrypted/coded sender units, so you don't have to break the seal to remove the gearbox. The white plastic connector on the sender unit signifies it's an encrypted sender unit, and matched to the tacho head, so the connector shouldn't be sealed to the sender. If you swap the sender, it'll trigger a security fault in the tacho head.
A basic road side check, is to unplug the sender and make sure it triggers the correct faults on the tacho head. If things have been tampered with, it'll often either not trigger the correct faults, or no faults at all.
You can also do a technical print out from the tacho head, which will show all faults that have been triggered, and when they were triggered. If the tacho head is full of faults, with no record of repairs in the maintenance history, it doesn't take a rocket scientist to realise somebody is doing something they shouldn't be.
Also with most modern trucks, the speed signal from the tacho is fed to the truck's drive control module, which will trigger various faults if the tacho speed varies greatly from the gearbox/ABS speed.
For speed limiter checks, VOSA have a little box they plug in to the tacho head that simulates reaching the speed limit. Examiner plants their foot on the accelerate, and increases the tacho speed. If things are working how they should, the engine should throttle back once the speed limiter speed is reached.
Tampering with tachos and speed limiters is no longer a straightforward thing, but it still happens.
The only reason more drivers/operators don't get done for it, is lack of resources. It takes a lot of effort to do it so it's near undetectable now, and if an operator/driver is going to that length, they're probably going to be picked up for other questionable practices.
*I say new, but they've been available since around 2000.