Whilst I’ve never seen under the paint the elemental symbol Ti (titanium) and C (carbon) are present on the down tube just above the Van Tuyl logo and from searching up some other Van Tuyl frames without paint it seems the builder has made a few frames this way. Pics for reference
This is a pure alu frame. Ti may (just may) refer to rear dropout and C is probable due to fork being made of carbon - one you shown couple of pics above is a totally different beast. Mono stays at rear, clearly visible Ti lugs etc etc
Nice builds.
Is that Van Tuyl really a titanium lugged carbon tubed frame? I may be wrong but it looks like alu to me.