The adapter in the example image represents the connection of a Guide TA435 thermal camera ballast. For newer devices, where our website does not mention a ballast side part, the ballast usually already has the M52 x 0.75 thread shown in the middle figure. In these cases, it is enough to buy the stirrup, but there may also be products, e.g. certain Pulsar products, where the ballast side part e.g. bayonet lock on the opposite side of the thread. In such cases, the required accessories are marked below the product. The clamping adapter can only be connected to the appropriate M52 x 0.75 thread on the thermal camera side. You have to buy the one for the diameter of the outer tube of the scope's front lens. As a reference, you can use the data in the following table, which opens by clicking: Rusan adapter / stirrup sizes according to scope types The table shows the scopes of different brands in order of name. The vertical columns show the possible outer diameters. Behind the name of the scope, the first column shows the diameter of the front lens, while the second column shows the diameter of the outer tube at the same point. The green cells confirm this second value, so the clamp size to be selected is the clamp. By selecting this from the size selector and checking the data with a caliper, we can select the appropriate adapter. The adapter shown in the middle picture is not part of the product, and must be purchased separately for a suitable device.