The ornamental design for an in-line anti-siphon device, as shown and described.
FIG. 1 is a front elevational view of an in-line anti-siphon device showing our new design, the top, bottom and rear elevational views being mirror images thereof;
FIG. 2 is a cross-section view taken along line 2-2 of FIG. 1;
FIG. 3 is a right side elevational view of FIG. 1;
FIG. 4 is a left side elevational view of FIG. 1; and,
FIG. 5 is a right side perspective view of FIG. 1.
The broken lines shown in the figures are included for the purpose of illustrating portions of the in-line anti siphon device and form no part of the claimed design.