A first shell 400 includes a first and a second base 410a, 420a arranged side-by-side, a center plate 430a, a first and a second inner plate 440a, 450a, and a first and a second holding plate 460a, 470a. The center plate 430a is arranged on the Z-direction side relative to the first and second bases 410a, 420a. The first inner plate 440a extends from a first end 411a of the first base 410a to a first end 431a of the center plate 430a. The second inner plate 450a extends from a first end 421a of the second base 420a to a second end 432a of the center plate 430a. The first holding plate 460a is integral and contiguous with a second end 412a of the first base 410a. The second holding plate 470a is integral and contiguous with a second end 422a of the second base 420a.