When I started building was toward the end of Loomis selling blanks and I didn't use G. Loomis stuff except for one IMX SJ6400 I extended to 6' (my take on Kirkman's ultimate smallmouth stream rod). Since they were the only one using those designations (maybe Shikari has some, only built a couple of those they also no longer exist), it seems the SJ and SB designations have fallen by the wayside.
The SJ series NFC X-ray series for example has an SJ 725 that is related as a medium heavy, but based on the specs is more powerful than the SJ 736 that is rated as a heavy power blank. It makes it harder to predict what your are actually going to get when you buy a blank without being able to handle it first. The reports I have read show many are confused by the rating system NFC uses for the X-rays in particular on various rod building sites. The SJ736 vs MB736 was a source of confusion for many, the SJ 703 is view as significantly lighter than its rating. I read A LOT of posts from people wondering about the ratings and their accuracy on the usual sites.