The law now says if you fit one of these frames it MUST have overrun brakes unless the vehicle is less than 750kg, but then it can only be used to tow to a point of safety.
Any recovery of a vehicle must only be undertaken with an A frame with overrun brakes.
The units used by the AA and RAC DO have overrun brake attachments, they also use heavey duty straps around the lower arms, these last for ages, the A frames that use chains on the lower arms can cause damage to the pressed steel and alloy arms
Avoid any A frame with pads on the front as these are very old units and are designed to push against the METAL bumpers of older cars, put one of these on a plastic bumber at your peril
You should also use a trailer board for lights etc, in line with the reulations, and the towed vehicle must be taxed unless on trade plates.
Most important of all, check that your insurance will cover this type of towing, after all it is only a trailer if it has brakes, so tell them what your are recovering to make sure that they understand that you are pulling another car without a driver
Hope this is useful I have used these frames for over 25 years without problems