Each STIHL model has a unique series number, which also makes up the first four digits of any part originally made for that series saw. Look on the bottom of the handle for the part number, and see what the first four numbers are, then look at this list, and you will at least be close to which saw you have. For the 1121 series, you may have a 024, 026, 026 Pro, or MS 260. The only way to tell for sure is to measure the bore, or look at other features, and even then it can be hard. At least with the series number you can look at the right parts list.
STIHL
SERIES MODEL
1106 090
1107 07
1108 08 S
1109 090 G
1110 041
1111 051, 075, 076
1112 041 G
1113 030, 031, 032
1114 020
1115 045, 046
1116 015
1117 042, 048
1118 028
1119 038
1120 009, 010, 011, 012
1121 024, 026, MS 260
1122 064, 066, MS 650, MS 660
1123 021, 023, 025, MS 210, MS 250
1124 084, 088, MS 880
1125 034, 036, MS 360
1127 029, 039, MS 290, MS 310, MS 390
1128 044, 046, MS 440, MS 460
1129 020 T, MS 200 T
1130 017, 018, MS 170, MS 180
1132 019 T, MS 191T
1133 MS 270, MS 280
1135 MS 361
1137 MS 192
1138 MS 441