The design is the visual features of the CASE FOR A SERVOMOTOR shown in the drawings, whether those features are one of shape, configuration, ornament or pattern or are a combination of any of these features. The stippled lines are provided for illustrative purposes only and form no part of the design. Variants are included. The sharp jagged lines indicate that the article varies in length, width and height within the break lines, as illustrated in the reproductions. Drawings of the design in forty-five (45) views are included, in which: Figure 1 is a rear left isometric view of the CASE FOR A SERVOMOTOR, in accordance with the present design, the rear right isometric view thereof being a mirror image; Figure 2 is a front view thereof, the rear view thereof being identical; Figure 3 is a right view thereof, the left view thereof being a mirror image; Figure 4 is a top view thereof, the bottom view thereof being identical; Figure 5 is a rear isometric view thereof in the open position; Figure 6 is a rear isometric view thereof in the open position showing the environment, i.e., the servomotor, in stippled lines and not forming part of the design; Figure 7 is a top view thereof in the open position; Figure 8 is a rear view thereof in the open position, the front view thereof being identical; Figure 9 is a right view thereof in the open position, the left view thereof being a mirror image; Figure 10 is a bottom view of a variant of the CASE FOR A SERVOMOTOR, in accordance with the present design, the front (and rear) and top views thereof being identical to Figures 2 and 4; Figure 11 is a top view thereof in the open position; Figure 12 is is a rear isometric view thereof in the open position; Figure 13 is a front right isometric view of a variant of the CASE FOR A SERVOMOTOR, in accordance with the present design, the front left isometric view thereof being a mirror image; Figure 14 is a front view thereof, the rear view thereof being identical; Figure 15 is a right view thereof, the left view thereof being a mirror image; Figure 16 is a top view thereof, the bottom view thereof being identical; Figure 17 is a rear isometric view thereof in the open position; Figure 18 is a top view thereof in the open position; Figure 19 is a rear view thereof in the open position, the front view thereof being identical; Figure 20 is a right view thereof in the open position, the left view thereof being a mirror image; Figure 21 is a bottom view of a variant of the CASE FOR A SERVOMOTOR, in accordance with the present design, the front (and rear) and top views thereof being identical to Figures 14 and 16; Figure 22 is a top view thereof in the open position; Figure 23 is is a rear isometric view thereof in the open position; Figure 24 is a front right isometric view of a variant of the CASE FOR A SERVOMOTOR, in accordance with the present design, the front left isometric view thereof being a mirror image; Figure 25 is a front view thereof, the rear view thereof being identical; Figure 26 is a right view thereof, the left view thereof being a mirror image; Figure 27 is a top view thereof, the bottom view thereof being identical; Figure 28 is a rear isometric view thereof in the open position; Figure 29 is a top view thereof in the open position; Figure 30 is a rear view thereof in the open position, the front view thereof being identical; Figure 31 is a right view thereof in the open position, the left view thereof being a mirror image; Figure 32 is a bottom view of a variant of the CASE FOR A SERVOMOTOR, in accordance with the present design, the front (and rear) and top views thereof being identical to Figures 25 and 27; Figure 33 is a top view thereof in the open position; Figure 34 is a rear isometric view thereof in the open position; Figure 35 is a front right isometric view of a variant of the CASE FOR A SERVOMOTOR, in accordance with the present design, the front left isometric view thereof being a mirror image; Figure 36 is a front view thereof, the rear view thereof being identical; Figure 37 is a right view thereof, the left view thereof being a mirror image; Figure 38 is a top view thereof, the bottom view thereof being identical; Figure 39 is a rear isometric view thereof in the open position; Figure 40 is a top view thereof in the open position; Figure 41 is a rear view thereof in the open position, the front view thereof being identical; Figure 42 is a right view thereof in the open position, the left view thereof being a mirror image; Figure 43 is a bottom view of a variant of the CASE FOR A SERVOMOTOR, in accordance with the present design, the front (and rear) and top views thereof being identical to Figures 36 and 38; Figure 44 is a top view thereof in the open position; and Figure 45 is a rear isometric view thereof in the open position.