@David I wouldn't get too caught up on when the change happened... there were at least a dozen things on my chassis that needed to be heavily modified, welded, remachined, or redone entirely that I couldn't find any information about on being a known issue. I could be mistaken but I believe #5 was part of the first shipment and it seems like every chassis in the double digits onward was pretty consistently identical. I only mentioned that info because I wanted to explain why I needed to use the middle and didn't use the ends, not to imply that you have something to worry about. I'm sure #261 is plenty new enough.
The tunnel cap pieces I have which were ordered about a year ago are 3.03" inside to inside.. so probably for a 3" outside tube.