Your rifle is not only rare, but it is rather interesting and could explain a great deal about Spandau during 1910.
Spandau is the most difficult maker to locate for the 98a, considerably so, - I suspect Spandau made very few 98a carbines, and 1910 is especially elusive, I think because Spandau went back the G98 early in 1910. Your rifle could be a good clue, or rather good proof of this observation.
First, I think your rifle is actually an Erfurt made rifle; I say this because of the firing proof, which is Erfurt's style, second because the b-suffix does not compare well with Spandau/1910 G98 production, but does exactly match Erfurt/1910 production. To confirm my suspicion we would need to see a clear photograph of the right receiver acceptance.
Although it is known that Spandau did produce Kar.98a receivers in 1910, so far none have been observed made (assembled) by them, though there are two unidentified reports, one from the KCN and another an old internet report. So far no confirmed Spandau/1910 (Spandau made, as in having characteristics that carried over from Spandau/1909 production) had been positively identified, just receiver shots.
Lastly, Spandau made about the same number of Kar.98a in 1908 & 1909, about 12,000 each by serial observation, 1909 is far more commonly seen. 1910 as above, receiver shots except for yours, which cannot be confirmed as to maker. A peek at Spandau G98 production shows that there was a sharp decline in both 1908 & 1909, but starts to rise in 1910 and doubles(+) in 1911. This could suggest SPandau made its move back to G98 very early in 1910 and made very few carbines in 1910.
Your rifle could go a long way explaining what really occurred, but I do not think it is at all reasonable to think this 98a is a Spandau assembled rifle, especially at half way through the b-block. That is about double the production of 1908 or 1909 and almost no Spandau/1910's are known.
Anyway, interesting rifle!