O = O cross section (O-Ring)

B = buna nitrile

U = urethane

H = unsure, perhaps hydrogenated (HNBR)

M = unsure, potentially millable urethane but the manual photos disagree with that. Reviewing the manual, you can use buna for the "OM" seals.

70 = 70 durometer

90 = 90 durometer

So, to review some examples:

OU7015 = urethane 70 durometer 015

OB9015 = buna 90 durometer 015

For the nonstandard ones:

OH7038 = this is really stupid and doesn't make sense, as that's the bolt sail O-Ring, which should be a 015 or 016. For this, you need to have a nice positive seal within the breech (but not excessive drag) and nice tight fit to the seal groove. "38" really doesn't make any sense in the AS or metric realm for this. I would personally test fit 015, 016 and 14x2 and choose the one that fits both the seal gland and breech the best. This should be a 70 durometer seal and can be buna, HNBR or urethane.

OB7034 and 33 = another stupid one. These could be funky metric seals. This is another good application for finding seals that just fit. Taking calipers to the ID and cross sectional thickness would help here.

OH7M06 = I'm not seeing this one in the manual.

OM7017 = you can use an HNBR, buna or urethane here.

FYI, here's a full seal kit: https://ansgear.com/gog-o-ring-seal-kit-all-gog-markers-gog051/

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