The unlatched Frasers I have examined have a groove about .050" deep in the left action wall to accommodate a leaf spring latch to hold the breech block closed. The latch is screwed into a groove in the left side of the breech block. I think this closing system was only used a little early on, as the wall of the receiver is left only a little over 1/8" thick. The oval latch Ben described is the most often encountered.
Many of the later Frasers had an intercepting sear, but the ones I have seen differ in their construction from the patent drawings.
