Item 40 is an alternative oil seal (without a retaining shoulder like the original Item 35 oil seal). Item 41 is the circlip which retains the alternative seal, because the seal has no integral retaining shoulder (the circlip should fit in the groove intended for the shoulder of the Item 35 seal). Therefore, you can use Item 35 or you can use Items 40 & 41 together instead.
In theory, you can replace the alternative seal without splitting the engine cases; Just remove the circlip, pull the old seal out (e.g. by using self-tapping screws), push the new seal in and then refit the circlip. To replace the original type seal (Item 35), the engine cases must be split because the integral shoulder prevents the seal from being pulled out.
Regards,
James