Welcome to the forum.
I have never actually seen a Beogram 1000 and to make thinks even more complicated B&O reuses their model numbers. I looked in the library on this site and saw a picture of a Beogram 1000 which looks to be the predecessor of my mid 70s, pivoted arm Beogram 3000. I also looked at a B&O site and found this: http://beophile.com/?page_id=8253
From what I can see Victor is correct in that the counter weight locks into position and is not adjustable. In my machine the weight is adjustable. the procedure is to set the tracking force to 0 the adjust the weight to balance the arm. Then the desired tracking force is dialed in. My guess is that in your machine with the tracking force set to 0 the arm should be close to balanced. One thing to note is that the mass of the weight is not evenly distributed about the arm but rather offset to the opposite side of the cartridge offset. This is important to properly balance the arm. The balance of the B&O is superb. Other arms with symmetrical counter weights such as the SME achieve the same thing with outrigger weights.
The best thing is if you can find the correct part, maybe from a junk machine as Victor suggested. If you are forced to improvise, set the tracking force to 0 and position your home made weight for the arm to balance. Do try to offset the weight to the opposite side of the cartridge.
Good luck and let us know how it goes.
I've recently bought a Bang & Olufsen Beogram 1000 from a junk shop. It's seems to be working ok which surprised me but it is missing the counterweight. Does anyone know where I might be able to get one? Or know of one from another turntable that might fit? Any help gratefully received! Thanks so much, Lulu.