It is a simplified ruleset for Squad Leader / Advanced Squad Leader.
Much of the ruleset is missing rules are missing, and you apply rules from Squad Leader or Advanced Squad Leader.
Retro is not a complete ruleset, you fill in the gaps with Squad Leader or ASL.
So I would take this line from the Retro Introduction to be a rule:
"If you are familiar with how Squad Leader is played, you will have no difficulty using this variant"
I would suggest:
- If Retro mentions it, fill in vital gaps with the minimum amount of rules from SL or ASL.
- If Retro doesn't mention it, use SL/ASL rules as an optional expansion to Retro.
- If there is doubt about interpretation of a Retro rule, use whatever is closest to the way SL or ASL does it.
- "Line Of Sight" is mentioned, but Retro has no LOS rules at all. So use SL/ASL rules or a subset thereof.
- "Minefields" are not mentioned, so minefield rules from SL/ASL can be used, but I would consider this an "expansion" to the ruleset, not part of the basic game.
- Broken units and Close Combat are mentioned, but there is no Retro rule for broken units in Close Combat. Squad Leader eliminates them instantly, so we do that.
Here are some of the missing pieces:
(Retro Tanks cannot rotate their turret, so we adapt Squad Leader facing and pivoting)
- Units with a covered arc (including Ordnance, Bunkers and AFVs) can cause Hesitation in their covered arc only.
- Vehicle Hesitation uses front or side armor values, depending on facing.
- Pivoting is not moving. You may pivot AFVs or Ordnance at no movement cost, during the movement phase only.
- Turreted AFV Firing during Fire Phase after pivoting within hex during Movement Phase +1 Column shifts right on Armor Combat Table.
- Non-Turreted AFV or Ordnance Firing during Fire Phase after pivoting within hex during Movement Phase +3 Column shifts right on Armor Combat Table.
- There are no Multiple Hesitation Rolls for vehicles.
- HEAT Gun Factor is caliber in cm round down. So 10 for 105mm. HEAT depletion is "H" on counter rear. H7 means your last HEAT round, >7 no HEAT available.
- HE vs Vehicle: as per HEAT, but a if you score a "Kill" on the ACT, roll a D6: 1= kill, 2-4 the vehicle must Flee in its next movement phase according to the Fleeing DRM rules but without the +3 DRM. 5= immobilized. 6 = no effect.
Comment: HE vs AFV, The Fleeing vehicle has been damaged or stunned, so the crew fall back, risking Hesitation and a second hit. This gives the SU-122 some chance against a PzKpfw IIIJ (50L) with just HE.
Squad Leader Radios:
ASL counters have a number, such as "8" on the front of the counter. Squad Leader counters, see table below. I would guess British Commonwealth and Japanese to be 7. Other nations as per Russian.