AS it is an antique in the US, and it's cartridge and date of manufacture make it an antique in Canada I would, contact the RCMP or your local Provincial registrar and have them generate an FRT.
It is an antique in the US, so just order it and have the dealer enclose the antique information for Canada, supplied by you and deal with customs when it arrives.
It will have a manufactures date based on a 1861 or a 1863 model and the rim fire or centre fire cartridge of .58 calibre would qualify it as an antique.
I have brought in antique 1873 Springfield's from the US and I have never been asked for an FRT number.