The eariest Marine Band SP20s used pin-like nails that went through both reedplates instead of screws (as did the Golden Melody and plastic-bodied chromatics at the time, so those would have to be tapped for screws of the appropriate diameter.
Winslow is correct about the early SP20s with drift pins. Two years ago I altered a Golden Melody with drift pins just using drill bits in pin vices and old Lee Oskar reed plate screws. It wasn't major work, but took a couple hours, since I don't usually do that kind of modification.
However, if your older SP20 has the larger screws, the thread I linked indicates that the new combs can still take the larger older screws.
If you buy the new reed plates you should order some of the smaller screws that they are made to take. ----------
Parts from several of the junk harps, like Piedmonts and Bluesbands will fit too, although Piedmont covers will taste terrible and muffle the sound and Blueband covers are sharp and pointy and mustache hair catchy, but if you ever need a SP20 comb cheapest way to get them would probably be to buy one of those harps. :)
I’m really glad all that silliness about the Sp20 has fizzled out. I still see the occasional isolated and misinformed comments but the sp20 seems generally to now be held in high regard again. I have examples dating from back in the 90s, and the most recent are every bit as good as the best I’ve collected across that time. I think it’s simply a great design, taking the reeds from a marine band and installing them in a reedplate of same thickness as a marine band, fastened with 6 bolts to a durable and consistently made comb which is easily cleaned, and covered with non-vented, smooth covers ideal for amplified playing using a cupped mic. The comb is keyed to encourage easy correct assembly, the comb includes a prop to support the covers against crushing. It has all the playability of a marine band with all the problems removed. All the problems except the vulnerability to reed breakage, but that seems to not bother some people. I guess some of us are just lucky?
I really like SP20s and see them as a bargain among the MB size Hohner harps. They play expressively with little effort. You listed the positives above.
The Marine Band basic is off my list of usable harps due to nails instead of screws. The Basic Marine Band costs too much to be disposable and I had my fill of nailed construction harmonicas years ago.
You mention the vulnerability to reed breakage of the SP20, and I agree with that assessment of MB sized Hohners in general. I can't speak to the MS series as I did not care for how they played stock, though my Low Big Rivers bought at SPAH on $12.50 clearance prices play OK after being put on Zajac combs
The easy response of the reeds in the Hohner Marine Band sized line (including the SP20 and Rocket) seems linked to their vulnerability to failure under hard play. That is why I also have a set of Manjis to use in situations in which I might exceed Hohner reed tolerances for breath force. I don't find Manjis to be quite as easy to play expressively, but they do hold up well and I keep working on them to improve their response. ----------