Is the hole too big for the 6-48. If so you might try an 8-40. I would definately agree with Jim, assuming originality it won't be Metric. Machine screw sizes start with 0 at .060" nominal dia & go up in .013" steps. Thus a #3 =.099", #5 =.125", #6 =.138" & #8 =.164". You can mic the old shank & it will likely be just a couple thousdanths below one of these sizes. Most all gunsight threads are in standard dia's but finer threads than "Hardware" standards.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra