According to the flier I have, the 1895G "Guide Gun" is specifically the 18.5" barrel in 45-70. It does not list the barrel as ported as I remember. I know it did come that way in previous years but they may have discontinued it this year.
The 1895 G is 4570, 1895M is the 450 marlin, then there a couple of other 1895 models also, mainly changes to stocks, finish and barrel length. Calibers are 45-70, 450 marlin, think they still make the 444, and I also beleive they have it in 44 Mag but I'm not sure.