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Turns out I must have a 1931-385 head on 1930-385 engine. The plugs on the head being used are 14mm, while the plug holes on the spare head are 7/8-18. The head being used is marked with an "HC" while the spare is not.

I know the motor was redesigned in 1931 to increase HP. I imagine that would include a redesign of the valves and head including the contours of the dome. Could a 1931 head on a 1930 block be the reason for the markings previously described?

29-36 valves are same. Hollander interchange manual lists 30-31 384 heads as interchangable. HC stands for high compression, if it's been cut it could cause valve interference

Thank you, Fred.

Still wondering about those plug sizes. An Autolite chart I downloaded says 1931 could use 7/8-18 or 18 mm or 14mm plugs while 1930 could use either 7/8-18 or 18mm. However, the '30-'32 Service Parts List says all 7th and 8th Series use the same plug which is identified as 5/8"-18. I've seen a reference to 5/8" nowhere else. I figure the Service Parts List would be more reliable than the chart as it relies on Series designations rather than the year, but don't understand the reference to 5/8"-18. 5/8" = typo?

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5/8-18 sounds like a typo to me. Attached is from a 1962 AC Division spark plug catalog.

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Given the plug data and looking at the '30 -'32 Service Parts List, it is clear the head presently being used on my 740 is 1932 or later. It uses 14mm plugs which were used from '32 to '36. It is stamped HC which corresponds with the 1932 high compression head (as opposed to low compression head) as set forth in the Service Parts List.

The SP List carries the same part #s for 1930 and 1931 384 c.i.d. heads, but the numbers for 1932 are different. Therefore, I am given to believe the heads are not interchangeable.

The 1932 (or later) head fits on the engine, so there must be something else about it that doesn't work. Perhaps the HC engine has a smaller chamber with a lower dome. This might account for the valve marks.

Whether the marks are a result of incompatible head design or over-planing, I guess the simplest solution would be to just use the spare head which has the correct size spark plug holes.

Just for the record I visited a gent in Seattle last summer who owns a 1930 740 roadster. I went to buy some parts and found out he's had heads manufactured for Packards over the years. He had a few new ones in his shop. I asked him about the marks. He told me they are a normal artifact of the factory's semi-automated machining out the valve chamber in the head and not an indication of any problem with the valves.