If you want a functional meter, don't waste your time looking for period camera-top meters... most of the Leica-meters for the LTM no longer work, and having them rehabbed is probably not cost-effective.
I would like to have have a functional Leica meter for my Barnacks, but so far have not found one in working order.
My everyday shooter is a III from 1934; I usually keep the 35mm lens on it, and have a zoom-finder "permanently" mounted in the shoe. I was carrying a Gossen Luna-Six meter, but objected to its bulk.
Last year, in a box-lot of meters from eBay, I discovered a Toshiba "Linear" CdS meter, which is delightful: small and accurate. That has become my user meter. A friend told me he thinks it's a copy of the Gossen Luna-Pilot.
While camera-top meters "look nifty", I think they would be a bother, and generally prefer to use my handheld. A built-in incident dome is a nice feature to have. If you're shooting scenes with really challenging light / detail levels, then a spot-meter is the safest way to go.
Prior to getting my Gossen and Toshiba CdS meters, I was using either a GE DW-series or Weston Master III meter, withe decent results.
CdS meters are nice as there aren't affected by cell deterioration over time.
Good luck !
LF