Section 415(c)(1)(B): https://www.law.cornell.edu/uscode/text/26/415#c
Retirement contributions for the self-employed individual are not permitted to be reported on Schedule C. Line 19 of Schedule C is only for employer contributions made to the accounts of non-owner employees. (Of course if you have non-owner employees other than a spouse, you are not permitted to have a solo 401(k), so in your case nothing is permitted to be included on Line 19.)
