[TonyJSenior]

Correct - on the isolated one, the replacement connects between the wiring attached to the two lugs with the center lug being the positive side as you said. As far as the others, the center lug is also positive as well, but you'll need to find a chassis point to connect the negatives of the replacement caps to. I've not seen a 610, but in the sets I have Philco often has chassis connection terminals riveted to the chassis underside that can be used. Usually those bakelite blocks have a chassis connection on them as well. Don't forget to disconnect the wiring from the old caps just in case they're shorted when installing the new ones