Ok here is the problem. Logitech rebranded their company to LOGI in 2015. So all products should have LOGI brand on it.

Since your M331 existed more than 6 years ago, i can only assume it is old stock. The new stock is probably the cut cost model. So they removed the rubberized coating. The LED light i assume is visible between the buttons?

Logitech has changed the sensor to non visible light type many years ago. When it first existed it was marketed as laser sensor. But its standard nowdays for almost all of their products.

On the unifying mouse, i believe they might have removed this feature on cheaper range because it allows people to reuse old hardware. But the newer expensive ones has it.

MCL is also a hardware distributor locally besides M-Link.