SE25 - 2GS
TR - Semi Deep
1L - Flap size. Flaps on 2Gs are slightly more forward compared to your standard SE models. The 1L is a shorter and less forward than a 2L. If you’re worried about DD growing out of the saddle, I’d skip a 1L all together and go for a 2L.
205 - as someone mentioned above, 5mm of foam added to the entire panel. This is a good panel for something basic built with a more slender shoulder. The saddle is a 2014, so you will likely have panel compression anyway which means it’s not going to fit like a NEW 205 and is likely more open in front.
34128 is the serial.
On the other 2Gs saddle you currently have - the above poster is correct in her decoding. The SE17 was know for having a “too flexible” tree back in the day, they changed their mix up of the carbon fiber & plastic material in 2014 and created the SE25/26. The S2 is a 17.5 and the SE17s many times rode a little bigger because of the way the trees broke in.
The panel on the SE17 also likely rides much wider upfront. Again, accounting for the flexibility of the tree, the year, and an additional 5mm of foam removed for the shoulder area.
Semi-deep saddles often feel like they ride .5 smaller, and it was pretty often I could compensate for a flap by adjusting the seat in my riders that went in half deeps when they were between sizes.
I’d be happy to look at a picture of your daughter in both saddles, though the only real way to properly fit a saddle to both horse and rider is in motion. The entire position changes. So video would likely be best, though I understand some people are uncomfortable with that. So just make sure you have good pictures at the trot and such too!