Diameter of bullet
This might sound a bit macabre, but what is the diameter of the bullet? From what I can see in the photo, it looks quite large - possibly 600/1000" Minie-style round used by rifled muskets and early breachloaders such as the Snider Enfield .577 or similar sized Martini-Enfields (both which were abundance in that region in that time)
If so, it means that Sir John Slessor was hit by a black powder musket round whilst flying a then state of the art aircraft! That would be a rare, possibly unique, event, akin to, say, a Typhoon pilot being taken out by a single SLR round.