I’ve always seen that hole, like there was something missing. What is it? What goes in the hold? Am I missing a part?
This is the accessory port on the old Manfrotto and Bogen tripods. It was designed to take a 3/8 screw, like what is on many of the accessory arms and heads. It is also where you can put the strap lanyard. Part R055,349 is a screw in with a metal loop. This can be used for multiple things, most commonly attaching a strap or a carabiner to hang you
camera bag. This screw port was found on all the 055 and 190 tripods. This is similar to the accessory part on the new MT190 and MT055; which has an anti twist feature when use with the Manfrotto mini arms. This new mini arm can fit in some of the old tripods, but like the picture above the hole placement is the limiting factor. If your hole is on the side of the main casting,. then you can use it or if you have any of the new Manfrotto MT190, MT055, MT290’s then you are guaranteed a fit. There are a bunch of accessories that go with these arms, you can find that page here.