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Just checked. Those are the belts I have and they definitely do not fit the grooves on the spindle pulley which is bone stock. They are definitely wider than the old belts. I thought that was because of 40+ years of wear. on the old belts.
I have only looked at a maybe a half dozen 10EE that were not seriously molested in past life, but I know they take an A belt, if you have doubts call Monarch. That said if the belt rides in the groove without bottoming out and does not shred and make a mess its probably okay. I am sure on my 56 the belt rides a bit high in the groove but that is okay compared to the opposite. Even though the lathes came with either a 3 or 5 hp motor I serously doubt most folks use more than 1 hp so if the belts work okay, don't shred and allow you to do the work you want to do I would not worry about it. I am not up on the later machines but a 70s machine still had A belts as far as I know.
At some point Monarch changed from the "A" belt to the narrower "3V" belt. The "3V" is what my 1977 10EE uses and I'm betting that's the case for your 1975. The "3V" is about 3/8 inches wide at the top but is NOT to be confused with the light duty 3L V-belt. McMaster Carr sells "3V" belts. Go to their website and look for "3V High Capacity Narrow Wedge V-belt." They come in both cogged and non-cogged versions and I don't know if one is better than the other for the 10EE.