Another one with an odd combination of proof marks. The intertwined TS was indeed used by Thieme & Schegelmilch of Suhl, Germany, but the only other mark visible on the barrel flat is that London preliminary proof. No final British proof anywhere, and no German proofs, either. Very odd combination.
I'll also second Raimey's dating due to the lack of a complete set of any country's proofs and the fact that it was retailed by Grundig (and built by T&S) in Germany. After 1893 the first German proof law came in to effect and it would have to have been proofed before it could be sold. The other way around, if it had been made in Germany before 1893 and then sold in England it would have needed a full set of British proofs.
The most bizarre thing is why on earth does it have that London mark which is just a preliminary?
And to answer the original question, no, it isn't nitro proofed. With all the other variables involved it's pretty hard to say whether it would be safe to shoot at all, but low pressure would certainly be the way to go if it is in shootable condition.