That is a typical Chinese Cheapo McCrappy universal pressure regulator.
https://www.ebay.co.uk/itm/Universal-Adjustable-Gunmetal-Fuel-Pressure-Regulator-For-Carburetor-Engine/123686056023?hash=item1ccc437057:g:BdwAAOSwNAtchgXBThey have no way of mechanically fixing them to anything unless you start making brackets. Than you may as well get a good quality regulator.
To adjust the pressure on these things, you push the top disc with the arrow down and then turn them to whichever position number you need.
However, you obviously have twin carbs fitted and I've never known a standard mechanical fuel pump deliver too much pressure for something like that. These pumps are designed to fill a single, stock carburettor with no regulator at all and only rely on the needle valve to shut fuel supply off.
If your engine wasn't running right, was it because it was over fueling because the floats in your carbs are not adjusted correctly. If they are shutting off too late, the fuel bowls overflow and it drips into the engine making it run too rich and lumpy.
By fitting a regulator, your mechanic may have just slowed the filling of the fuel bowls down so much that it not overflows. So what happens when you go flat out now? Have you tried it? Does it start to misfire after a while? If so you may be running too lean because your not getting enough fuel because it's been regulated down too much.
Your first step is to check the floats are adjusted properly and than go from there.
Frank