Without seeing pictures it's hard to say, could be a 311, 312, or maybe an earlier model. Here is a picture of one of my 311s and it just happens to be a slat bottom. I don't have a pic of my 312 but it would look like the 512 in the last picture only with 2 less bottoms. The 311 was fully mounted and the 312 was a semi-mount. The 309/309A are 3pt mounted and were made for the 504. I have a 309A but no pic of it currently.