I've got a '54 Merc convertible and faced the same problem. I found this seller on epay who had the right shocks, but I didn't save the label on the box. I believe they are Gabriel and the number on my shock is 739034. The vendor at this link was very responsive and the price was good.
Oops, I can't seem to get the link to post. PM me and I can send the link.
Note: when fully extended mine measure 21", not including the mounting studs at each end.
Marc