Hey - I know that gun!
In stock MKIII configuration, I like the 18.5# as it seems to result in less "hammer chatter" (there I go making up words again

) on the narrow rib that's left on the pickup rail after it's machined for the wide firing pin block sear lever.
Yours having been converted to MKI configuration, Either rate will function with 124's, but the 18.5 may be on the ragged edge of too much for the 115's. (Depends on the make of 115's, of course)
Hope you enjoy the pistol!
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