ranges. Using dyes to test various plastics for leaks, they found a suitable plastic called Durex 1661½, which could be easily molded into a case. By the spring of 1956, Aerojet had a near-final design. It was awarded a pre-production contract for 1,000 M18A1 Claymores, designated T-48E1 during testing. The initial versions of the mine used two pairs of wire legs produced from number 9 (3 mm) wire. Later when production was ramped up, the design was changed to flat steel scissor, folding-type legs. Early pre-production mines were triggered using a battery pack, which had been used with the M18. This was found to be undesirable for a number of reasons. Bill Kincheloe came up with the idea of using a "Tiny Tim" toggle generator, of the type used with a number of Navy rockets. Originally an aluminum box was used to hold the generator. Later a Philadelphia company, Molded Plastic Insulation Company, took over the manufacture of the firing device for the first large-scale production run producing a plastic device. The sighting for the device was originally intended to be a cheap pentaprism device, which would allow the user to look down from above and see the sight picture. After locating a suitably low-cost device, the engineers found that fumes from either the C-3 explosive or the cement used to glue the sight to the top of the mine corroded the plastic mirrors, rendering them unusable. They adopted simple peep sights, which were later replaced by a knife blade sight. Testing concluded that the mine was effective out to approximately , being capable of hitting 10% of the attacking force. At , this increased to 30%. The development project completed, the Aerojet team sent the project back to Picatinny. The Arsenal bid it out to various component suppliers. In 1960 it was type standardized as the M18A1. It was first used in Vietnam in the spring or early summer 1966. Minor modifications were made to the mine during its service. A layer of tinfoil was added between the fragmentation matrix and the explosive. This slightly improves the fragment velocity, and protects the steel fragments from the corrosive explosive. A ferrite choke was added to prevent RF signals and lightning from triggering the mine. Variants M68 Inert Training Kit The M68 kit is designed to familiarize personnel with the placement and arming of a real M18 directional mine. It comes