“X-brace” - How old is this mod?

I’m looking at a PA-18. The last log entry for fuselage fabric is ceconite in 1969. The fabric appears to be in decent shape, and I’m well aware of the fact that ceconite from the 60’s can still look great if taken care of as our family has one covered in the 60’s and still looks brand new still. However, this PA-18 has the top deck X-brace installed of which I thought could only be done during a recover (maybe not?). If it can only be installed during recover then I’m curious as to when guys started installing the x-brace? When did Altee develop the STC? This could help me determine if the fabric has indeed been done since and perhaps no log entry made. Thanks!

You can weld pretty much anything on the fuselage of a covered flying Super Cub, depending on where you’re welding will determine how much fabric cutting or disassembly is required.

Isn't there paperwork detailing the x-brace install?

I've welded in several on covered cubs.... you just pull the fabric back on the one corner in rear