If you cannot find the Yamaha part number or confirm the fork manufacture,
Are there any identifying numbers or brand name on the seals, if there is, take a note, measure the inner diameter the outer diameter the seal height and count the number of sealing lips, normally one but can be two, then call up a seal manufacturing supplier give over the info and ask them for the best option, there is a whole industry out there that specialise in supplying seals for industrial rams of all sizes, the bonus could be that you get to find an industrial version of the seal that might be cheaper than the Yamaha or pattern supplied part
http://www.athenaparts.com/eng/products/view/8419/fork-oil-seal-mgr-rsd-32x44x10-5
I think this is the part a 1981 DT 175 MX uses, the DT is listed on the, View all couplings tab,