Back in the late 60s and 70S I used to rebuild a lot of FEs and SBF engines, and every once in awhile you would run across one that had lash caps. In school we were taught they were to reduce valve stem wear from the rocker arm. The metal for the lash caps was harder than the valve stem. Metallurgy has come a long way since then, so has the quality of
oil. In those days if an engine made it to 100,000 miles that was really good.
I forgot about lash caps until I bought a 1971 302 last year that had never been torn down. Still had original bearings, bore, rope rear seal and the heads did not have hardened seats. The valves were original FoMoCo. It had lash caps on all the exhaust values. The valve stem tip was machined smaller so that when the cap was on, it was the same diameter as the rest of the valve stem. The lash cap only pushed on the top of the valve stem.