+1
I found only picture of model.
http://scalemodels.ru/articles/9122-amodel-1-72-jak-28im.html
On internet is lot of photos of Yak-28I variant but not IM.
“CAS” is a stretch
That is, if at all. I think that UB-16 was used in this version only with combination of S-5P/P1 chaffs same as in case of Yak-28PP.
https://ruslet.webnode.cz/technika/ruska-technika/letecka-technika/a-s-jakovlev/jak-28pp-brewer-e-/
https://www.airwar.ru/enc/spy/yak28pp.html
I will post it in Russian. You can translate it to English. But same thing you have already in description.
С сентября по октябрь 1969 г. проводились заводские испытания самолета Як-28ИМ, оборудованного четырьмя подкрыльевыми пилонами для дополнительного вооружения. Опытную машину построили путем доработки серийного Як-28И №4940503. Совместные государственные испытания бомбардировщика проводились в 1973 г. К тому времени в крупносерийное производство запустили Су-17 и Су-24, оснащенные большим ассортиментом вооружения, и продолжать работы по расширению боевых возможностей Як-28 уже не имело смысла.

This source was used then by other websites.
Thank you.
I even wanted to get the Yefim Gordon book, but it looks like it would be a folly act.
I was thinking of going to central air force museum and trying my luck to ask them about it, but I do not know when I’ll be able to do this.
You can try luck but its pretty rare plane as only 1 Yak-28I was transformed to use reportedly 4x UB-16.
Yak-27 can be pretty decent CAS.
https://ruslet.webnode.cz/technika/ruska-technika/letecka-technika/a-s-jakovlev/jak-27-flashlight-c-/
+1, its always good to have diversity
Images of it having ground attack armament are missing.
There is even less information on it than on the Yak-28IM.
One would think that since it went into mass production, there would be more information about it.
This is not true, this modification never existed.
- Have you found any more references to this modification? If not, it is the Yak-28P
You mean: if not so it is Yak-28I (not P variant). We discuss here difference between Yak-28I and IM.
Open the sources section and read, please.
Each one contains a mention of this aircraft.
Based on these sources there was a Yak-28I equipped with rocket pods. How many (2x or 4x) is unconfirmed.
To be honest, most if not all internet sources probably refer to the same book Авиация и время 98. We will definitely need more sources. Preferably also with some picture.
+1 why not