Micro Seiki BL-51 and Which Tone Arm?

You won't have any problem mounting an MA 505 on a BL-51.
The manuals at Vinyl Engine shed light on the situation:

Micro MA-707 pivot to spindle: 222 mm (effective length - overhang)
Micro MA-505 pivot to spindle: 222 mm
Infinity Black Widow pivot to spindle: 222 mm
Grace G-704, 707, 714 pivot to spindle: 222 mm

Check the arm length. I was a dealer for the tables but it has been a long time. Still have a blank arm board for them. There should be a site that gives arm length. I would think that the arm would be perfectly compatible, I may have used one on the 51 but after 25+ years I don't remember. It might have been on the DDX-1000. I had at least one in those days and it was a good arm.

I looked up the arm review from 1977 and it was highly recommended by HI FI Choice, which was in those days doing excellent tests on components. It was a quarterly and each issue would be devoted to testing one type of component. They recommenced the arm for use with tables like the DDX-1000,Technics SP10 MK2, and Lux 131, which were outstanding tables of the day. The mass is listed at 14-16 grams and cartridges with a compliance between 8 and 15 are recommended. Most of todays cartridges should work. It was an expensive arm in those days, costing 90 pounds, compared to 52 for the Audio Technica 1009 and 35 for the SME 3009/2. I might try a Denon 103 in it, they are available for $150 and are very good, I was just listening to one.