There are two shaft configurations: non-split ST1500, or split-shaft ST1500-"S".
There are two different trimmer heads: bump head (original), no additional marking, and the rapid reload head, indicated with an "F" at the end of the part number, e.g. ST1500-"F".
The 1501, 1502, 1503, 1504 indicates different kits, specifically the different battery sizes.
The other Ego system that does trimming is the PH1400 (multi-tool head), which works with the STA-1500 trimmer attachment. You can buy those pieces individually, but much better to buy them in the kit. MHC1502 (edger plus trimmer). Another kit has the pole saw added too.