Yeah, that was back in Dec, 2014, long before I could build up the database....had to rely on other sites and other people's research and take their word at face value that spring loaded pins went into '51. (We all know how that sometimes turns out.)
Fast forward to now and the data says '50 is the likely transition date. Now the data may be incomplete and '51 may be the true transition timeframe. If could be that the transition may have begun in '50 and continued into '51 with both styles used concurrently as stocks of firing pins, bolts, retaining pins, extractors, and bolt carriers were used up. I'd expect to see some as-issued '51s with spring loaded FPs though. To date I can't find a single one.
Post up a pic of that sneak again. Let's take another look at it maybe there's something we didn't notice back then that might shed some light into it now...
Edit: Found it: http://sks-files.com/index.php?topic=632.0
ЗЮ prefix. I don't have any others in the database. I do have one yet to be entered and this is the GB listing. While the receiver cover isn't shown, the seller ID's it as a '50 (presumably off the receiver cover):
I think '50 is much more likely than '51 LC.