Locking sleeve

The design is the features of shape, configuration and ornament of the portions of the LOCKING SLEEVE FOR SECURING A DEVICE TO AN ENDOSCOPE as shown in solid lines in the drawings. The portions shown in stippled lines do not form part of the design. The dot-dash-dot line does not constitute a feature of the design. It is included for illustrative purposes only to define a boundary of the design. Drawings of the design are included. FIG. 1 is a front, right, top perspective view of a locking sleeve for securing a device to an endoscope showing a first variant of the design; FIG. 2 is a rear, left, bottom perspective view thereof; FIG. 3 is a front, left, bottom perspective view thereof; FIG. 4 is a front elevation view thereof; FIG. 5 is a rear elevation view thereof; FIG. 6 is a right side view thereof, with the left side view being a mirror image thereof; FIG. 7 is a top plan view thereof; FIG. 8 is a bottom plan view thereof; FIG. 9 is a front, right, top perspective view of a locking sleeve for securing a device to an endoscope showing a second variant of the design; FIG. 10 is a rear, left, bottom perspective view thereof; FIG. 11 is a front, left, bottom perspective view thereof; FIG. 12 is a front elevation view thereof; FIG. 13 is a rear elevation view thereof; FIG. 14 is a right side view thereof, with the left side view being a mirror image thereof; FIG. 15 is a top plan view thereof; FIG. 16 is a bottom plan view thereof; FIG. 17 is a front, right, top perspective view of a locking sleeve for securing a device to an endoscope showing the second variant of the design with a cylindrical rail member inserted therein showing the environment of the design.