i don't own one of these but there are 2 piecing pins front and in the lets call it a plug that tighten everything together
i am guess there is no seal in the front because the seal in the back is just a urethane Oring being it is green
https://www.pyramydair.com/blog/201...Q9XI-qMS0HmBL_ORzuHJJAn5BvA53fS9p0QZsMUZ449U3
so, if that is true the size Oring is a 113
shop around
https://www.theoringstore.com/store...ct_info&cPath=367_1321_1324&products_id=27507

and you right Umarex is useless

but i could be wrong