Bentwood Rocker

I have a bentwood rocker that I've owned since the early 1970's. While in the process of sprucing it up a bit, I decided to do some research on its markings. The only photo I've found that is exactly like it is on Pinterest, which led me to a website that is longer in business. So here I am.

I've scoured this chair from one end to the other and back, and there is nothing indicating it is a Thonet. There are, however, markings on the bottom of both cross bars which connect the two bent pieces of wood which form the rockers (see 3rd & 4th photos). The best I can determine, the markings appear to say "X-59 X-99" . I've been all over the internet and to the library and can find nothing to explain what this is.

The chair has a double caned back and a grid is woven into the caning on the seat, representative of a Thonet design.

Is there anyone out there who might be familiar with, or otherwise shed some light on the markings on this chair, or perhaps direct me to a source that may?

I would appreciate anyavailable information. Thanks!