low gearset 904?

Pretty sure all of the later versions of the 904, which are 998 & 999 transmissions had the low first gear sets. To be able to quickly tell if they are lockup or non lock up, look at the input shaft. The non lockup will have an input shaft that is the same size all the way to the tip, like a 904. A lockup will have an input shaft that is stepped down to a smaller size at the tip of the shaft. If you can find a core, you may get all of the parts cheaper than buying new and just take it to your transmission guy and let him harvest what he wants/needs to fix you up.