Does it show if you leave oil film on it? Maybe use wax like on stock, I use Minwax paste floor wax.
I have a '51 Tula that doesn't show that stripe - or on the 'spare '51 cover I have.
Here's another SKS forum -> http s://sks-files.com/
and maybe something on Yooperjohn's site?
Boris ??
The stripe spot is where the cross bars sit in the factory crates. It's normal and not something worth changing. All the non refurbs I've seen have it.
I looked on the SKS-FILES link I sent you and the #1 post refers to "? Factory blemish ?". In Post #14 Boris B has really good pics of the 'stripe' they refer to, and suggest those w/o the stripe have been "Permablued" . I've not seen that product around NB/NS shops but there aren't that many out here anyway. Here's a link that may be the solution you're looking for. You might want to search the SKS Files and others for use of it.
http s://www.bing.com/search?form=MOZLBR&pc=MOZI&q=permablue