Murnay: googled some more and saw that in another book, Förster says that he was one of FW's guards in Potsdam, i.e. Leibgrenadier ... so I guess there's one more thing we know about him: he was probably quite tall.
And one more thing from Förster: a 1727 letter from Eugen to Seckendorff, which already talks about the 1000 ducats per year for Grumbkow and discusses how he would get the money, speculating that he might not be happy to be caught with freshly pressed imperial ducats, because it would be easy to guess where they came from. Also, same letter references an imperial medal set with diamonds that Gundling got. (Good for him I say.)
Re: Rottembourg's bribery attempts
And one more thing from Förster: a 1727 letter from Eugen to Seckendorff, which already talks about the 1000 ducats per year for Grumbkow and discusses how he would get the money, speculating that he might not be happy to be caught with freshly pressed imperial ducats, because it would be easy to guess where they came from. Also, same letter references an imperial medal set with diamonds that Gundling got. (Good for him I say.)