Happy day...thank you for the courtesy of a reply.  I believe that a 401(k) has two money streams going to it:  the employee deferral and the employer contribution.  The client is excluded from an additional $19k deferral because it was taken from the W2 job.  And I understand the point about the partnership needing to be the source of the retirement plan.  So on the worksheet, Part II adjustment I back out the K1 Box 14 SE income, leaving just the Sch C income to be used for the 401(k) calculation, which reduces the allowable contribution to $17k. 

This is the example from the IRS link I included in the original post:  

Example 1: In 2019, Greg, 46, is employed by an employer with a 401(k) plan, and he also works as an independent contractor for an unrelated business and sets up a solo 401(k). Greg contributes the maximum amount to his employer’s 401(k) plan for 2019, $19,000. He would also like to contribute the maximum amount to his solo 401(k) plan. He is not able to make further elective deferrals to his solo 401(k) plan because he has already contributed his personal maximum, $19,000. He would also like to contribute the maximum amount to his solo 401(k) plan.

Greg is not able to make further elective salary deferrals to his solo 401(k) plan because he has already contributed his personal maximum, $19,000, to his employer’s plan. However, he has enough earned income from his business to contribute the overall maximum for the year, $56,000. Greg can make a nonelective contribution of $56,000 to his solo 401(k) plan. This $56,000 limit is not reduced by the elective deferrals Greg made under his employer’s plan because the limit on annual additions applies to each plan separately.

Any thoughts with what the IRS posted?  I know we can't rely on IRS publications for support...but I haven't seen an exclusion on my fellow Proseries users 🙂