There are a number of older Leica lenses in M mount that are actually LTM lenses with a factory installed M mount. You can remove a tiny screw and carefully break the adhesive (by turning I suppose) to remove the M mount part, leaving you with the LTM version of that lens. In every case, there is a native LTM version of this lens that was also produced and sold, but because the native LTM versions were produced and sold in smaller numbers. Thus, they seem to cost more on the market due to rarity. Also, the M-mount versions of these lenses had markings on the barrel for closer focusing (often 0.7m vs. 1m on the LTM version). Meaning....a purchase of the old M-mount lens, then removing the M mount, and ending up with an LTM lens is a seemingly good value. I've only done this with the 35/2.8 summaron. I've seen 21/4 super angulons with this configuration. Have not seen the 21/3.4 with a removable M-mount though.
And, save the M mount. Clearly! It may come in handy in the future with either your own M-mount body, or if you sell said lens; a wider market of M and LTM users.