Thanks again for everyone who reached out to offer their thoughts. I wanted to follow up with how I solved the problem. It might be useful for someone facing a similar issue in the future.
For clarification, my L1200 came to me in excellent condition. There was no sign of damage or abuse.
The first thing I checked was the parallelism of the negative and lens stages in isolation, with the head detached from the column. I tested this in a number of ways, and the stages were consistently parallel with one another.
Therefore, I deduced that probably the column was not perfectly perpendicular to the baseboard. As one member had suggested, the weight of the enlarger might have compressed the baseboard over time, causing the column to lean.
After much experimentation, I ultimately found a set of shims from McMaster-Carr (part number 97235K808) with slots cut into them that fit nicely around the L1200's baseboard bolts.
0.02" thick shims under the front bolts brought the head carriage/column into alignment. I double-checked whether the lens stage was still in alignment. And it was.
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