Steph,
The drop grate linkage isn't shown in the plan view, your correct, the tube in the plan view is the brake linkage, in the side view you can see the drop grate linkage and I'm assuming it's tubular simply because it has a dash- dot-dash line down the centre which I always thought represented a tube in a GA drawing?
If it's a blow down linkage then why not present on the GA, the only linkage present is the damper linkage, there being no linkage for a blow down valve, I'm not saying it's not, just cannot see it.
I've attached where I think the revised damper linkage would run, there being a lay shaft across the frames at the throat, it would be no more convoluted than the existing linkage but importantly not jammed between the firebox and frames. All hypothetical of course as I've no clue exactly what it does
and as yet have not found an image in detail of that area, I'm sure the answer is in one of the tomes on the shelf.