This is bracing that is needed for a shear wall. This provides lateral support, which is needed in areas prone to hurricanes and earthquakes. Typically, the plywood sheathing and drywall is all that is needed to provide shear support low risk homes. This kind of bracing is often turned on its side and let into the stud framing (notching the studs) rather than being placed between the studs and further braced above and below. There are also newer versions that are T shaped pieces of metal (maybe 10' long but only 1" wide or so), where the base of the T is inserted into a small notch that is cut into the studs and the top of the T is nailed flat to the face of the stud.
Note that the other bit of blocking at the top of your photo appears to be for the knob and tube wiring, which I hope you're in the process of removing/replacing.
See also: What is a shear wall and what is it for?.
Wikipedia entry on shear walls.
I agree with Jimmy Fix-it (in a comment on the question): residential sprinklers will protect best, if you have a good (dependable) water source. They are relatively inexpensive, because the sprinkler lines can be run from the nearest water line in the house provided the domestic system can supply the flows needed (13gpm to all design sprinklers and 18gpm to any single sprinkler operating). You don't need a separate waterline, riser, fire department connections, etc. However, be aware that when your insurance salesman says you'll save money in house insurance, you should ask him how much to add "water damage" if they accidentally go off.
But, to answer your question, the Code divides construction into 5 categories:
Type I and Type II: These types are "non combustible ", which primarily refers to concrete and steel construction. The main difference between I and II is whether the exterior and interior bearing walls need to be protected longer… it's 3 hours for Type I and 1 hour for Type II.
Type III is masonry construction using combustible inner structural members, or "ordinary construction".
Type IV is "Heavy Timber" construction. This is wood (fire treated and untreated) that is primarily thick (8" diameter!) timbers for structural members with masonry for load-bearing walls. Partitions (non-load bearing) walls can still be 2x framing.
Type V is anything or everything else… including wood framing, straw-bales, etc.
However, all these materials are divided into sub-categories of with and without fire sprinklers. There are many many requirements with each category… like everything, it's in the details. (I.e.: non-bearing wall construction, enclosed shafts, attic construction, etc.)
In addition to what you called "fire susceptibility" construction, the code identifies "flammability " of materials. All materials fall into 3 classes: Class A: 0-25 flame rating, Class B: 26-75 flame rating and Class C: 76-200 flame rating. Most often this is what kills occupants of burned buildings. Usually, victims are dead long before the building burns up and falls down on them. Fire sprinklers will protect against this too.
I'd also invest in the best fire alarm system (with battery backup)… and some come with additional horns and carbon monoxide detectors too.
Best Answer
Those are galvanized steel brackets attached with timber screw fasteners.
As neither of these products were manufactured in 1902, it is safe to say that someone in the recent past added them to reinforce the structure.