To Pac Rotors
Yes it is highly possible that there was upto 30 lbs(I think its pounds but could be kilos)in the tail, this weight ,as you say, can be added when the avionics/news gathering/mission kit is such that the c of g suffers. If you look at the 600 virtually all the weight that can be added, pax bags etc is in front of the mast. The weight is fitted to bring it back within the realms of workable c of g, and is added under a collar inside the tailboom just forward of the rotating dustbin,on a specific mounting for the purpose. And the internal collar ensures airflow thru the tube is not effected.
So it wasn't really a strange thing that it had the weight in the tail, in actual fact I think most of them have, and must have if fitted with the amount of kit this particular 600 had.
Regards
hoverbover