I tried to get some information about IS symbols and emblems, as well as Kasarov tried too, but we didn't find anything satisfying, that's why we decided to work around this symbol with doublechevron. SneakEye is also using icon from vanilla IS with crossed grenades above chevron in his All-Units mod.
There isn't, really.
The regimental insignias in the British army are quite visually complicated: they look like heraldy, often with similar silhouettes only distinguished by the details. That'd make them very difficult to turn into simple unit icons. There are proficiency badges meant to indicate special training, but they're usually lettered.
If that's the approach you want, you could take the leaves from the proficiency badge and use it as a universal "Specialist Infantry Section" symbol in the same way you suggested using the Chevron.
However, purely visually, I think SneakEye's approach of making them up is best. Given Assault Sections and AT Sections aren't even the same unit any more, you could even change the icon entirely. Go with crossed Sten Guns and a crossed Boys AT Rifle and AT Grenade or something.