I see many people don't use a space between a numeral and an abbreviation, such as "7lb" or "5mm". Shouldn't it be "7 lbs." and "5 mm"? Otherwise, a baby may be confused to weigh "71 lbs." rather than "7 lbs." Also, is there a period used after millimeter? (Ex. mm.)
I don't get the thing about pounds. if you read it
71 bs it doesn't make sense. However, there should always be a space between the number and the unit to prevent confusion (although I can't see how there would be confusion in then case you stated).
As to the period. The rule of the thumb is that metric measurements should never receive a period and US customary measurements should always receive a period (so,
*5 mm. would be incorrect).
I hope this answers your question.
In our journals we always use a space between the number and the unit of measurement. And we do not use periods after units of measurement, whether metric or not. It is a matter of style, not grammar.