Each chain link equates to about 18 degrees. As miles pile up, each pin in every link begins to slowly wear. Multiply this slight amount of wear by 134 ( number of links in the chain) and you have a measurable amount of chain wear. As Pat mentioned, machining of the head will also contribute to lateness of the mark. Multiply the amount the head was machined by two since the chain goes up then back down and the distance becomes shorter on each leg. Chains sprockets also wear and contribute to the lateness a bit. Earlier shop manuals address the condition. They explain that off-set woodruff keys are available to correct the situation, but then go on to explain that it will most likely not make any noticeable difference on how the engine runs and performs. In short these engines are designed to accommodate some lateness in the valve timing. Never advance the valve timing in front of the mark, not even a slight amount. You are fine with your slightly late timing. Only when the lateness approaches 7 degrees or more do I become concerned. Usage of off-set keys becomes tricky and confusing. The cam timing becomes corrected but the index marks stay late. Keep things like you have them, you will be fine.
Some perfectionists will use off-set keys, dial indicators and valve timing specs. to get exact valve timing. But as the early manuals state, "it will probably make no noticeable difference". Lets face it the valve timing is only exactly correct for a short period of time, then it begins going late every time the engine is run.
