Trader, the new pictures tell a whole lot more!
For one thing it seems the interrior of this inner structure may have been put together out of older salvaged timbers, which is evidenced in at least one empty mortise hole being in the "wrong" place; uprights being crudely joined where mortise and tenon would have been used earlier; cut outs on uprights that are out of place; etc..
As well it's clear now that this inner structure was never filled with chink but was meant to be open; making the outer part of the structure a necessity. In all it does seem that some of the other posters are right and this is a newer structure, but in this case one built out of older salvaged wood. Because of this, probably what dating that can be done regarding existing iron from the oldest timbers would not be conclusive of much.