Roblox’s shader source, prior to October 2016, was present in the client in the DirectX HLSL format - from playing and toying around with it, it’s clear that prior to January 2020 (PBR introduction), specular and gloss (or rather, SpecularIntensity and SpecularPower) was the system used for the specular highlight appearance.
If you find a client from around May 2013 (when the shaders were rewritten to support Dynamic Lighting), up until that point in 2016, it might be worth a look just to gain a fuller understanding of how things worked on the rendering side during that time.