I just examined a compact microwave oven and it had an odd pair of openings at the magnetron waveguide end. The waveguide funneled out and then the larger of the two openings was half covering the lower portion of the funnel and the second opening, about level with the magnetron, was almost a slit of 1/2 x 2 inches. Both aperatures were cut into the sheet metal wall and niether was as large as the usual single waveguide opening of about 3 x 2 inches I am used to. I want to couple to this waveguide with another waveguide but this geometry is confusing. I feel like opening up the larger aperature and closing the smaller one before proceding. What does this odd configuration mean?