You got any digital calipers? Measure the external shaft diameter at the hosel.
It's probably a 0.355 inch tip. If not, it's a 0.370 tip.
You can get C-Tapers in 0.355 or 0.370.
0.355 is the most common for irons, 0.370 for hybrids.
The difference is more significant than you might think - around 0.4mm - so don't get the wrong shaft.