I once had the mind to fit a std. Mercury trigger block to a Challenger, replacing the ugly contrivance of the Challengers, in a bid to make for the best Mercury ever, but never could fall in love with the Maxi rail and left it.
The Mercury S was probably the best BSA springer ever, outshooting even the Goldstar.
Despite the Challenger gaining the Maxi rail, I never had any trouble mounting scopes in the std Mercury grooves, at least not enough to justify the ugly plastic on the cylinder, or the holes drilled into it.
My Lincoln Jeffries might give it an argument but lack of scope provision would leave it just behind.