According to the Technical service manual for a 1980 (I have an 80 but a lot of the parts are '79) the identification is as follows.
there is a tag on the shift control housing. lacking that since you say you have to press down to get reverse, and its up and to the left that would indicate a SR-4. Also how many bolts are in your shift lever housing? the SR-4 has 4 the other 2 have 6.
R 1 3
_ 2 4
although pictures would verify this.
My first ever Resto project.