What is the difference between the arsenal slr107 and slr04 besides caliber?
I found a handguard that I really want to purchase. The description says

"Be sure to choose the correct thread type for your AK (Most Krink Types us 24mm)

-  For SLR 107 Krink Types and SAM 7K-01, 04,44,UR, SFK, 104UR, 106UR users choose Mod1 mount Stamped Receiver or Mod 3 Mount Milled Receiver

-For SLR107 Krink Types and SAM 7K-01, 04,44,UR, SFK, 104UR, 106UR users choose SAM 7K Railed Gas Tube if you want to use our gas tube

-Krink Type users will have to remove HG retainer use use this Handguard"

The way I'm reading this, its saying SLR107UR, 104UR, 106UR, SAM7k-01, SAM7k-04, SAM7k-44 all use mod1 for stamped receiver (SLR UR models) and mod3 for milled (all sam7k variants)

That last sentence, though.  having to remove handguard retainer on a 104UR.  I wouldn't touch any handguard that required doing that on an arsenal.

I've been seeing this reply on other forums also. But why is that? Can't I just send it to a smith that's good with AKs and have them put on back on if I decide to? This is a very handguard from a reputable company.

The handguard is from SLR Rifleworks for anyone wondering. One reason people are steering you away from it is that the handguard retainer on the AKSU pattern rifles are specific to that rifle, so you're going to have a hard time finding a replacement if K-Var ever sells out of them, which is often for most of their AK parts. It can take years for them to re-stock some items. I would say if you're going to do it, to go ahead and get the retainer now from K-Var to keep on hand since they have them in stock:

https://www.k-var.com/retainer-assembly-for-lower-handguard-for-krink-arsenal-bulgaria

Good advice thanks