Ditto on the reason for the external line. It's a quick way to compensate for clogged internal oil passages, and there's not much of a downside to the change, other than being easy for one of us oldtimers to spot. Most folks who didn't grow up around stovebolts won't know the difference.
The compression pressure readings you got are OK, but you'd probably get a little higher reading with all the plugs out and the throttle blocked wide open. Don't sweat it, just do it differently next time. Be sure and adjust the valves before you check the compression- - - -a tight valve adjustment can affect the readings more than leaving the plugs in. As long as your compression readings are within 10% of each other, actual pressure numbers don't mean much. You can get totallt different readings, pressure-wise, from different gauges.
I usually plug my compression gauge into the shop air line to check it for accuracy occasionally.
Jerry
