LLoyd3,
Was the barrel marked 8/88R or did you measure a chamber cast? If it was marked 8/88R (including the R), I suspect it stands for 8mm model m88 rimmed(8x57IR). Normally the R wouldn't be marked as the rim recess would show it is a rimmed cartridge. If it was proofed before 1912, but after Mar 1893, the bore diameter should be shown in gauge measurement as 172.28; in which case, 8/88, 8/M88, 8/M88R could have been added as a voluntary mark to designate the cartridge it fired. I could find no 8x88R cartridge in either Dixon's book or the Municion.org website, but if you measured the case length to get the 88, you may have included the chamber leade of an 8x75R Behrs, by mistake. I hope this helps.
Mike
