Ruger didn't drill and tap their Mark II autos as early as the 215- prefix, so I think that ones been done after-factory.
The first D&T guns that came out was the Camfour MK-512CA
From the reference...
MK512CA (Ruger model 0199) Special Distributor model. Has smooth laminated grips, is drilled and tapped for scope base and comes with a scope base and rings. Known s/n range 225-46986 to 225-57468. A total of 302 produced in 2003 for Camfour (MA).
This model was apparently pretty successful from the D&T standpoint as Ruger decided to offer all guns with adjustable sights as Drilled and Tapped (a great advertising tool!). The earliest of each model known are as follows, again from the reference (note how rare some of these D&T models are!):
Drilled and tapped receiver MK512, MK678, MK678G, MK10, KMK512, KMK678, KMK678G, KMK10, P4B, P512 and KP512. In 2004, all regular production adjustable sight Mark II pistol model variations were drilled and tapped by the factory and shipped with a Weaver style scope base adapter. MK512 S/N 225-57739 is a prototype of this feature (probably an overrun of the Camfour issue described above) and pictured in the 2004 Ruger catalog. For the regular production models, all of which are identified by a circled "B" on the box label, the tapped receiver begins to be seen by at least s/n 225-82223 (although a run of KMK678G in the 225-82023 to 225-82103 s/n range have the circled "B", the boxes were marked in error as the pistols do not have the drilled and tapped receiver). The estimated quantities produced of each regular production Mark II Target pistol with drilled and tapped receiver are as follows: MK512 = 5,650; MK678 = 850; MK10 = less than 200; MK678G = 500 or less; KMK512 = 2,200; KMK678 = 950; KMK10 = less than 200; KMK678G = 400; P4B = 1,000; P512 = 7,000; KP512 = 600.
The last D&T model to come out was the USST model KMK512GCUS (Ruger model 0168) Special Distributor USA Shooting Team model. Known s/n range USA00001 to USA02599.
And yes, 226-18143 is the highest Mark II pistol I've heard of. It is the highest known 1 of 1000 KMK41K which was Ruger's commemorative and final model to the Mark II series. Being a Standard, it is not drilled and tapped.
Chet15