1.36J! Get 'im @Rogerborg!
Also, whoever said you can make the TM MK23 better for less than a Novritsch SSX23 is mistaken. An SSX + suppressor is circa £150, a stock TM is the same. The upgraded inner barrel, bucking and TDC will likely cost you another £50 or more on top. Plus the SSX23 has a nice metal slide and fibre optic sights.
I'm no Novritsch fan boy but think it's a bit silly when people pipe up about building a gun yourself for marginally less than he's selling some of his. Especially when you're talking for your son, unless you specifically want the learning experience associated with working on them.