You should consult with a trademark attorney offline.
I am not your attorney, and my responses on Avvo should not be considered legal advice.
That is too close in my opinion to register M33Ms for clothing. Jewelry is a different class, and they may allow it. You should check with a trademark attorney to do a clearance.
I am an Attorney-at-Law, licensed to practice law only in the state of California. Unless we have both signed a formal retainer agreement, you are not my client, and my discussion of issues does not constitute legal advice. Opinions expressed herein are those of the author, and do not necessarily represent the opinions of those who hold other opinions.
Generally, plurals are infringements of singular terms. What you propose seems a clear case of deliberate infringement. The test is whether people would be confused into thinking that there is some association between your M33Ms jewelry and M33M clothing. In fact, that is so close it would almost certainly be deemed willful infringement and subject you risk of treble damages and attorneys fees and an injunction. Jewelry and clothing go together so closely and M33M is so fanciful that you have no believable defense for such a willful and wanton infringement as you propose. In fact, this public website is so highly ranked by Google that the brand owner will now probably be on the lookout for any Hawaiian knockoffs of its M33M products.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
What you are seeking is a trademark clearance opinion. This should be entrusted to an experienced trademark attorney. A number of factors must be considered - these include the strength of the existing mark as applied to the particular goods/service (M33M seems both uncommon generally and arbitrary and entitled to a high level of protection, for example), the audible, visual and overall impression of the proposed and preexisting marks, the nature of the goods and services offered under the proposed and existing marks, the channels of trade used to reach the relevant consumers (in this case they would appear to be the same or overlapping channels even though the products themselves are different), and the sophistication of the consumers (retail consumers of clothing and jewelry being considered the least sophisticated, generally).
Even if you succeeded in registering a mark, at the US PTO, you could still be confronted by the owner of a pre-existing mark and accused of diluting the existing mark or engaging in unfair competition. For all of these reasons, it makes sense to review your situation with trademark counsel and obtain a written opinion letter.
One letter almost never makes a difference. The test is whether there is a likelihood of consumer confusion. Because apparel and jewelry are closely related, your attempt to use an established fashion trademark to sell jewelry would almost certainly lead to a cease and desist letter and/or law suit for trademark infringement.
Also, you seem to have some fundamental misconceptions about trademark law. Trademark rights arise from use in commerce, not registration. Registration has many advantages, but even if you succeed in obtain a registered trademark, it does not prevent your from being sued for trademark infringement if someone else claims to be the "senior user" of the trademark (or a substantially similar trademark).
In a situation like this, it is almost always a very bad idea to use a trademark that has only a one letter difference. The law does not allow one company to benefit by associating itself with another famous brand name----you can't take advantage of the hard work that another company did to develop its brand name, and then trade off the same name. You need to go back to the drawing board and come up with something original.