Most European-style timber workbenches (now the most popular style of self-made of woodworking benches) use a bolt system to fasten leg sets to stretchers. The main advantage is that you can nudge the bolts up tight again if anything loosens due to changes in humidity.
The solid brass nuts measure 25 x 25mm and fit perfectly in a 1in (25.4mm) pre-drilled hole in the stretcher, distributing pressure evenly to prevent breakout. A bench can be made very rigid (without complex joinery) by using a wide stretcher, since the stretcher is bolted into position.
The steel bolts are a massive 152mm (6in) long and 12.7mm (1/2in) diameter - a lot bigger and heavier than the photo suggests.
The bolts are sold as a pack of four.