Although I've been buying and restoring vintage dirt bike for decades I'm a bit of a newbie to 70's enduros. I've been surfing the web for months now sourcing parts for my CT2 (I know, wrong thread but I'll bring it back around). The CT/AT1s and CT/AT2s and CT/AT3s share a great many parts and many have different part numbers but still fit all other models. My question is about oil pump covers. The CT/AT1 pump has the part number 248-15416-01-Y4 and the CT/AT2,3 has the part number 3V6-15416-00-00. So what is the difference?
Well, I guess you could say it is like a belly button. The "Yamaha" logo is either an "innie" or an "outie"?
I've seen "outies" that have 248-15416-00 stamped into the inside and I've seen "innies" with the 31400 stamped on the insides. I've seen covers listed on ebay that confuse this logic ("innies" with the 31400 stamp listed as CT1/AT1 covers).

So, am I correct in my assumption that the CT/AT1s have "innies" and CT/AT2,3s have "outies"?

I've also noticed that all clutch covers for sale are stamped with the max oil capacity which matches the spec listed by Yamaha. CTAT1/2/3 covers are all stamped with "750cc", '74 DT100s are stamped with "650cc", etc. The cover that came off of my running CT2 looks identical to all others I've seen (even has the 31400 stamped inside) but the oil capacity stamped on the outside is "600cc". I can't find any similar model with that a .6 liter oil capacity. Do the guys on the Yamaha assembly line like to occasionally slip in a "one off" just for fun or is this something that others have noticed?

Sorry, my assumption put forth in my last post was reversed. The correct assumption (question) is:
Do the CT/AT1s have "outie" oil pump (and clutch) covers and do the CT/AT2 & 3s have "innies" oil pump (and clutch) covers?
Outie = raised Yamaha logo
Innie = inset Yamaha logo

Your later assumption is correct - the early bikes have raised letters, '72 and newer have recessed letters. Same applies to DT/RT models.