Hmmm, not sure what the utility is of that buckle? I use a Timbuk2 bag every day that doesn't have that kind of buckle and can't imagine what I would use it for. I guess it could be useful to disconnect the strap so you can attach the bag to a rack of some sort, but I always wear my bag over my shoulder.
I use the Tenba insert that andersju posted above and it's a very nice insert. I will say, though, that while I find it perfect for a full-size dslr and lenses, I've grown to prefer the thinner Domke inserts for RF and even TLR stuff. The Tenba is a large insert with thick padding, which makes the bag seem bulkier. The Domke inserts are thinner and work better for me for small RF lenses, and one of their inserts fits a TLR very nicely. It's not as protective but I'm pretty careful with my bag. To me the Domke inserts are very pricey for what they are, but they do work and I haven't found a cheaper alternative.