On the topic of cataloging Machins- my suspicion is that many in North America rely on the listings in Scotts to catalog. I am also assuming that people here are only interested in cataloging on the basis of value, color and maybe bands.
If that is the case, in my opinion, Scotts is hopeless and virtually guaranteed to frustrate. The listing are in essentially in order of date of issue. There are these little squares or color scattered about. So, finding where a particular 20p stamp goes requires scanning over the Scott's listing for the 85th time. I understand the frustration. Its no wonder given 39 trillion different Machins having been issued since 1967.
Alternatively, if you were to invest in a copy of Gibbons Concise (easily obtained from
Amazon.co.uk) you would find, wonder of wonders, an index of Machins by value-an amazing break-though in philatelic technology! With this index you look up the value, check the color and the catalog number is indicated. Its really simple and takes little to no time. Its easy. Really.
If you still want to know the Scott MH number, that is now made easy. Since the SG listing gives the date of issue, looking up the equivalent listing from SG to Scotts is simple.
Just my thought on how to lower this frustration level.
-Larry