I believe lexmark didn't get rid of the SDL connector until shortly after they took over production from IBM. That said, the ones made in the UK (Greenrock) are very different than their brethren across the pond. Check, for example, if your spacebar is ABS or PBT: I have read that many are ABS. Given that, and other differences, I would say it's not too unheard of to find one with an SDL connector. IBM keyboard differences are very erratic. The terminal 'boards, especially, are very different (on the DT wiki, it says that the 122-key F's all have model numbers like 611034*, whereas I have a 122-key 6110866 that came with a 3179. I would also like to mention that John Elliott's pages also feature a 6110866, so I'm somewhat confused as to why the DT wiki is wrong)
That said, I'm not an IBM expert by any means.
I'm glad you got your keyboard. Make sure to post more pictures of it.