I don't think you'll find an FRT for that - unless it has a manufacturer engraved into it somewhere it'll be tough as there are almost no records on turn of the century producers - many were part of larger manufacturing conglomerates or made guns or parts on contract for a larger wholesaler. Generally each maker had some sort of distinctive appearance of their parts because of their skill set, the intended quality or manufacturing process.
That said, if you can prove it won't chamber any of the no-go cartridges (based on what will fit, not what it's nominally chambered for) and has a proof or markings that date it to pre-1898 you don't need an frt number and can get an antique letter.
From appearances I would say it's probably a good quality belgian gun, or possibly french? I haven't seen a ton of independently produced french guns so it's a tough call... I'd hedge my bets on belgian
My thoughts are:
1873 MAS style ejector
1873 MAS style loading gate
Belgian bulldog style trigger guard - both shape and fitting
Grip - angled like a pinfire, but profiled similarly to the pryse.
Above average belgian style cylinder, frame and barrel
- Frame is a little more contoured than your average bulldog - a lot more along the lines of the 1873
- Barrel almost looks like a pryce
- Front sight looks a lot more like those seen on belgian pinfires
- Cylinder - slightly more contoured than the usual with fluting rather than dished walls. Similar to British RIC's and pryse guns of the era.
Considering that the belgians produced both contract guns for other manufacturers/arsenals and also made knock-off firearms of questionable quality it doesn't surprise me that this gun shows characteristics from several different guns. My general thought is that it shows a lot of early cartridge gun designs - pinfire, 1873 mas, pryse... all of these fell out of favor as better designs and faster manufacturing processes were developed. Then again... maybe the maker built this later on using designs that were no longer patented to avoid legal infringement.... oh the joys of antiques
End of the day only way you'll be able to prove its status is by showing a pre-1898 proof mark and proof it can't chamber no-go cartridges. Hope this helps!