Arm question

My turntable has rather an odd arm on it. From the pivot forward it looks like a Rega RB250 but the counterweight and stub is completely different. The very end of the stub is labelled as an Audio-Technica AT1005/II and the stub is chromed metal rather than plastic. The counterweight is also chromed with a small knurled locking nut. The Audio-Technica arm is J-shaped (thanks to Peter Stockwell for the info) and looks nothing like the arm I have.

I'm assuming my arm is an RB250 with a modification similar to the origin live counterweight mod but using the counterweight and stub from a AT1005/II.

It doesn't look like an amateur job - does anyone know of any firm doing the modification.

Generally the mod appears to work well however the counterweight is too heavy for light cartridges to be used. With the AT110E (7.2g) its fine although the weight is quite far forward. With the MC1-Turbo (4.1g) I can only get around 0.5g of tracking and the anti-skate is way out. Any suggestions gratefully received - I'm considering purchasing another cartridge and this limits my choice.

Regards
Steve