Hello Mitko and welcome to the Forum.
I have a little bit of information that I can add to my last post, although unfortunately it is not directly related to your query. I have some Le Rhone engine numbers for 150 Squadron RAF in 1918. At this date these would have been 110 hp Le Rhone 9J engines rather than the earlier 80 hp Le Rhone 9C. Most of the numbers have five digits, for example 29642 an 44509. It is not shown whether these were French or British built engines. However, there are a couple of engine numbers with four digits only, namely 9195 and 9403, and these are described as “Spare French Le Rhone (engines)”. If nothing else, this at least shows that there were some Le Rhone engines with four digit numbers.
The Le Rhone 9C engine had a nominal power rating of 80 hp but a maximum power output of 92 hp. This may explain why the Bulgarians recorded it as a 90 hp engine. If this engine came from one of the RNAS Nieuports then it would have been from the Nieuport 11 rather than the Nieuport 12. As mentioned earlier, the Nieuport 12 had a 110 hp Clerget engine.
A red ring around a white circle was indeed an early from of National identification marking used by the RNAS, in 1914-15. However, I would be very surprised to see such a marking in 1916. Some Nieuport 12’s were built in Britain but the Nieuport 12 serial 8913 was a French-built aircraft. Most French built Nieuport 12’s operated by the RNAS retained their French roundels (on the wings – there were no roundels on the fuselage).
Hope this helps,
Regards,
Mike.