Incredible, qbteachmt. Thank you for taking the time to explain it all.

Yes I am preparing both the individual and the company returns. I did not prepare the W2s. The difference between $6750 and $4500 was because the client sent me his 2020 5498 and not his 2021. 

Based on all this information, it appears that this is an employer sponsored plan and the $4500 was a pre-tax contribution and it "manifests" in his return in the form of lower reported gross pay. The explanation on H&R Block's site says that the amount listed in Box 12 with an "s" code was "not included in box 1".

So I'm confident that the client is getting the tax benefit of the contribution.

Thanks again.