Think I have an SBA

I have an alto with ser. 518XX Which on saxgourmet dates it to 1954, but I though the Mark VI started in '54. I want to know is there any more value in this being a later model SBA? I would think the metal might be closer to a Mark VI metal. Anyway, Its been sitting in its case for the past few years and I've been toying with the idea of selling it. The engraving is sharp, but the laquer is in such good condition, I have a feeling it might just be a good relaq. Other than that, and a missing key guard (alt f# key) the horn is tip top. On saxgourmet's value guide I saw a mark vi alto is like $6100, and a SBA lik $4600, I was thinking I'd split the difference to $5350 then deduct 25%+100 for the relaq and the missing key and ask for around $4000 any insights into this horn or other ideas as far as pricing or where to sell? It's very pretty.