diff --git a/EIPS/eip-663.md b/EIPS/eip-663.md
index fe42c2b768ffac..feae741cab8b9e 100644
--- a/EIPS/eip-663.md
+++ b/EIPS/eip-663.md
@@ -47,7 +47,7 @@ This results in wasting a byte in the cases of only referring to the top 255 sta
 
 ### Option C
 
-This option extends Option A with two new instructions, `DUPSn` and `SWAPSn`, where the value of `n` is encoded as an 8-bit immediate value following the opcode.
+This option extends Option B with two new instructions, `DUPSn` (`0xb2`) and `SWAPSn` (`0xb3`), where the value of `n` is encoded as an 8-bit immediate value following the opcode.
 
 The value `n` has a range of 0 to 255, but otherwise the same rules apply as in Option A.
 
@@ -68,6 +68,12 @@ This has no effect on backwards compatibility.
 
 TBA
 
+## References
+
+A similar proposal was made with [EIP-174](https://github.com/ethereum/EIPs/issues/174). Read the thread for some detailed discussion.
+
+Rootstock [RSKIP26](https://github.com/rsksmart/RSKIPs/blob/master/IPs/RSKIP26.md) also introduced `SWAPN` and `DUPN` with Option A described above.
+
 ## Copyright
 
 Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).