There was a square charge (7 1/4 x 7 1/4 x 1 3/4 inches) with four magnets but it only weighed 5 pounds. I don't think it became a standard Engineer charge.
Jock Lewes (SAS) devised a charge comprising a ration bag, PE, incendiary material, primer and dual ignition for the attack of stationary aircraft. Recipes varied but PE(including Black PE) was a stock item albeit in short supply and effectively reserved for SF. There is a mention of the Lewes Bomb (so called) in an official publication:
Royal Engineers Training Memorandum No 3, 1942. Lewes is not mentioned and Nobels 808 substitutes for PE2/3/3A .
Old thread but I want to add that I have that training pamphlet around here somewhere. Small, shows a diagram of the Lewes Bomb (they don't call it that) with dual time pencils, dets, which I think was a standard redundancy engineers used for demolition. But it mentions, I am paraphrasing from vague memory, that "units in the field report using single ignition and 100% success."
Success when correctly assembled anyway. One of the SAS books talks about the raid by unit leader David Stirling, Blair "Paddy" Mayne, and two troopers, where they parked their two jeeps outside the airfield, three walked in and placed 40 bombs on planes. They came out and stayed to watch the fireworks. They counted only 18 explosions. Checked some remaining bombs and realized some failed due to damp primers. I assume that meant they were guncotton pellets? So they decided to go back in with the jeeps and attack the non-burning planes with their machine guns. This was successful and it caused Stirling to decide to change tactics, not using jeeps just for transportation but to add more guns and rear gunners to them to conduct gun attacks, not sneaking around placing time bombs. This is the story anyway.
I believe the Lewes Bomb was one pound 808, quarter pound thermite, and used motor oil kneaded in which was Lewes' invention to slow the burn enough to light gasoline on fire instead of the explosive blowing all the oxygen away and not lighting the gas. The objective was to create a HE and incendiary to blow a hole in the gas tanks and light the gas so the plane would destroy itself. But it had to be in a small package so a man could carry 12 or more in a rucksack. The sappers told them that was impossible, they would need two separate bombs which would be heavy. Lewes had the answer.
It had a 1 ounce primer with detonator in the explosive ball (I wonder if the oil made it black?), a short piece of Cordtex went to that, the assembly was put into a ration bag with Cordtex sticking out, taped to another det which was on a 2 or 3 inch length of safety fuse, the fuse being inserted into the fuse snout of the pencil. Safety fuse offered a short amount of time in case the pencil tripped early, for the operator to pull it apart or throw it. Pencil was then taped to the bag, which was smeared with sticky tar. This was then wrapped in a cloth and put in the rucksack. At the target the pencils were activated early or at time of placing the bombs. The operator would take off the protective cloth and toss the bomb up onto the bomber or transport's wing above its tank. Or place on the seat of a fighter. The tar would keep the bomb from rolling down the wing or flying off during an explosion on the next plane. Very successful tactic until the Italians and Germans started posting guards at each plane. So one can see why jeep attacks with up to four Vickers Ks shooting 1,000 rpm each was a better choice.