To add a little more clarity to Andy's post, the 8SGA is an older set which uses a large size module - they've been out of production so long that I think you'll be hard pressed to find a new module for it anywhere. (I have a 72MHz FM module that's just gathering dust - you're welcome to it for the cost of shipping, but you'd have to get it converted to 75MHz to use it legitimately for surface applications.) The part number is FP-TF-FM.
The Prism is a much newer set and uses 2nd generation (more commonly available) modules. SPECTRAs come in both 72MHz and 75MHz models so you need to check which one you have. (The module should have a sticker on it that indicates what frequency band it runs on.) 75MHz SPECTRAs are getting hard to find, but are not all gone yet so you'll probably have better luck getting this radio in order for sub use.
Both these sets use "negative shift" to generate the signal they send, although as Andy indicated most modern (generic) receivers are compatible with both.
Kevin McLeod - Oscar II driver