I just got bit on this POS too. The thing that miffs me the most is that this crucial detail (the round cap only) is buried on the site. If you don't expand the product details which are hidden by default, then you don't know. So much like OP, I bought one because it's advertised as drop in. Got it in the mail today, excitedly start to install it...yeah. Major fail.
So, I just ordered the regular MOE MLOK, which would have saved me $5 to begin with and I WOULD have ordered had it been obvious the SL was incompatible. As for the $40 lump of incompatible plastic sitting on my counter? Well I guess I can request a refund, minus the cost of shipping to return it suppose. Meaning that when it is all said and done if I return this POS then I paid an extra $10 or so for regular MOE handguards.
Why for the love of god didn't you choose to make it ultra obvious that this had very limited compatibility on the site?????????????????????????????????
I shouldn't have to expand the "features" to discover that one of the "features" is that it isn't compatible with MOST mid length rifles.