Any I made and sold had a full set of stress calcs and from memory 10.9 high tensile, but that was my bumper, any manufacturer should have the bolt size and grade in the instructions, as they should also have a file with the info in incase of any insurance claims.
I was lucky as a good friend can do the calcs and produced the correct paperwork.
In my experience I have seen bumpers fail but also a chassis come apart as it had been poorly repaired, they are not rocket science to make but like most things a no win no fee bloke can upset the apple cart very easily. Remember the bolts should not be studs and should not be threaded all the way up to the head as these are weaker
A local lifting gear place can easily test it for you for a few quid.
The big issue is are they lifting equipment or not? But that's another can of worms, you see a winch with a load rating but not a bumper?