Here in the States, with a few exceptions, rectangular bolts gave way to round bolts much earlier than than in the UK, during 1850's- 60's. While there may be certain advantages with rectangular bolts such as increased contact area, I think when it comes to shear or bending strengths in bars of equal area, they are not so much different. In fact, depending on the shape of the rectangle and the type of attack, probably not as secure. For example. take a 16" tall door where the weakest point is the locking bolt itself. A single 2" round bolt has roughly the same area compared to a 12+" X 1/4" flat bar. Mosler actually made a safe like this not that long ago. Using one of the specialized pry bar devices, seen on Safeman's website, attached at the top of the door, what will happen? With the centrally located round bar, the door will start to bow out at the top, but remember in this scenario, the locking bolts are the weakest point, meaning the door is strongly enough built that the flexing is small and will focus energy of the attack on the locking bolt itself. With the flat bolt, bending and/or tearing will begin at the top of the 1/4" thick flat bar long before any thing happens on the 2" round. Only as the rectangular shape increases in thickness closer to the thickness of the 2" round do you see an increase in strength against this type of attack. I find it a bit hard to believe, that back in day the when construction costs were not the main concern in building the best burglary safes possible, rounds would be favored over flats purely for their economy. Granted I am not a safe engineer, just throwing out how I see it. Doug