It was "manufactured" at their Hephaestos mobile space station, but according to the Dragoons themselves (in Wolves on the Border iirc) the station was more a refit or maintenance facility than an actual factory. I take that to read they couldn't produce the components (fusion engine, gyro, etc.) there, but what they could do was take Archer skeletons and rebuild them to ARC-2W standard.
Jaime Wolf himself told Minobu Tetsuhara that all Archers among Wolf's Dragoons were of this signature variant.
The ARC-2W assembly line died along with the entire Hephaestus Station in January 3028.
It's noteworthy that according to the MUL, the ARC-2W refit was only introduced in 3010 whereas the Dragoons had arrived in the IS in 3005 already. By 3010 they had ended their contract with House Davion and had moved on to serve House Liao, fighting Marik forces in aggressive campaigns. I seem to recall they tracked Marik raiders to Scarborough, crushed them there and salvaged boatloads of stuff. Maybe the assembly line was built using salvage from that campaign.
Another thing that is of note is that Jaime Wolf, back in his Clan days, piloted an antiquated Archer in his Trial of Position.
Further, he acquired a Thor as isorla from Clan Nova Cat in an early battle. It so happens that the Thor B configuration features two LRM-20 and two SRM-4, the exact same weapons layout as the ARC-2W.