TBH I have never seen plated 12.9s and mostly that grade has been present in commercial production machinery such as injection mould machines etc far removed from outside atmosphere or at least inside a fully covered and lubricated enclosure ie and engine.
So it may be that the longer 'life' of the 8.8 combined with fine thread and plating ability gives a useful characteristic for the caliper application. I'm wary of very fine threads in aluminium, even hard grade as it can burr or get damaged too easily and then fail. Coarse and fine are OK. Steel is good for the ultra fine, even cast iron is a little too coarse grain wise for ultra fine threads.
I'd guess the upshot is you can use either grade but be especially aware of the plating on 10.9 plus maybe a 'shorter' working life. The old molybdenum steel alloy m/bike frames from the late 50s' used to wear out after 3-4 years.....as they hit the cycle end of life part of the stress/strain....and would then fracture!.
Here's some 'stuff' found on a fairly good (even if U.S.) site.
Metric bolts use their own numbering system as well, unmarked bolts are often used for light-duty automotive applications such as plastic retention or used with body retaining clips. Metric bolts have their own rating system based on Mega Pascals (MPas). A Mega Pascal is equivalents to about 145 PSI. Grade 8.8 bolts are manufactured from medium carbon steel that is quenched and tempered. Bolts (below 16 millimeters) have a yield strength of 640 MPa (92,800 PSI) and a tensile strength of 800 MPa (116,000 PSI). Heavier duty Grade 10.9 bolts are made of quenched and tempered alloy steel and have a yield strength of 940 MPA or 136,000 PSI and a tensile strength of 1040 MPa or 151,000 PSI. The premium metric bolts used in the most critical stress areas are Grade 12.9. They are also made of alloy steel that is quenched and tempered and have a yield strength of 1100 MPa or 160,000 PSI and a tensile strength of 1220 MPa or 177,000 PSI. Metrics will sometimes be made more identifiable by a red coloring, but finding the head mark on the bolt is the only way to be sure it is strong enough for your application.
Stainless steel bolts are usually made with a steel alloy that contains up to 19 percent chromium and 13 percent nickel. Standard thread stainless bolts are rated as 18-8 and have roughly the same yield and tensile strength as a Grade 5 bolt. Metric stainless bolts are rated as A-2 and are rated slightly under the standard metric 8.8 ratings.
When replacing nuts, washers or lock washers, it is essential that you use hardware of the same grade or higher to ensure the bolts can still meet the specified strengths for each grade. A Grade 8 bolt can be compromised when held in place with a grade 5 *US or nylon-insert style nut. When the original nut or bolt is missing, assume it is at least a Grade 5 *US or metric 8.8 when selecting replacement hardware.
- See more at: http://blog.hemmings.com/index.php/cate ... 0QKQH.dpuf
I also note that some companies are supplying stainless caliper bolts. Now A2 (304) is actually about 6.6 grade and A4 (316) a bit better. If that is strong enough? well, 10.9 is overkill. I also found references to 8.8 grade bolts relating to Bendix calipers for vehicles about the same age as, but not specifically the BX.
How about all those lovely Pozis the Japs used on crankcases on their m/cycles in the 70s? Nightmare to remove.